Which came first, the rattle or the rattling?

Whether or not you’ve seen one in person, you’re probably familiar with the way a rattlesnake uses its rattle. When cornered, a rattler takes a ready-to-strike posture, stiffens its muscles, and quickly vibrates its tail. Of course on the end of that tail is a rattle, or as the authors describe it in a recent paper published in The American Naturalist, “interlocking segments of keratin that fit loosely within one another.” (Keratin is a protein that also makes up your hair, nails, outer layer of skin…and a snake’s scales.) The distinctive rattling sound that’s made when these segments rapidly vibrate against one another causes many animals (including humans) to instinctively freeze and slowly back away hoping to avoid a nasty bite. It’s a very effective warning signal.

western diamondback

A western diamondback rattlesnake (Crotalus atrox) in Arizona, rattling its tail to scare away a perceived threat – presumably the photographer.
Photo from Allf et al. (2016)

Warning Signals

It turns out the behavior of tail rattling is much more common than the rattle itself. Rattlesnakes belong to the Viparidae family of snakes, all of which have long hollow fangs that inject proteolytic (protein-degrading) venom into their prey victims or some other animal stupid enough to mess with a rattlesnake. The rattle on the tip of a rattler’s tail helps it send a clear warning to stay away to avoid those fangs. A second family of snakes called Colubridae contains about two-thirds of all snake species, and they’re generally harmless to anything that’s too big to strangle. This is why you’ll find many kinds of small colubrids—kingsnakes, corn snakes, garter snakes, etc.—in any pet store that happens to be into the whole reptilian thing.

I owned a California Kingsnake (for 21 years, she passed away this past April) and I had seen her rapidly vibrate her tail a few times when she seemed to feel threatened. As someone who studies marine invertebrates, I’m not much of an expert on the biology of snakes. But I did know that kingsnakes don’t have rattles on their tails like rattlesnakes*, so I was a bit puzzled when I saw my kingsnake “rattle” her tail. It made me wonder whether the behavior had evolved before rattles themselves, but my wondering pretty much stopped there and I went on with my own non-snake research.

IMG_1022

The late Irma (1995–2016), my California kingsnake (Lampropeltis getula californiae).

Now researchers at University of North Carolina have done much more than just wonder about tail-vibrating behavior in non-venomous snakes. They subjected the idea to some hypothesis testing, showing that the action of a snake “rattling” its tail did indeed evolve before the rattle itself.

Reconstructing the Past

To figure this out, they took a phylogenetic tree (like a family tree, but representing the evolution of a bunch of different species) of snakes from a previous study and mapped the behavioral trait onto the tree. This statistical method, also known as ancestral character state reconstruction, uses some fancy math to calculate the likelihood that a given trait existed in the common ancestors of living species.

So let’s take a look at the phylogenetic tree in Figure 2 of Allf et al. (2016)1. First of all, each name on the right represents a different species of snake, and the circle next to the name shows whether that species vibrates its tail (white) or does not (black). Notice that all but three species included in the tree exhibit the behavior of tail vibration; even vipers that don’t have rattles and the non-venomous colubrids. Only the group of rattlesnakes has a rattle on its tail. At first glance it seems obvious that the behavior is older than the morphology, but it’s important to test these things statistically… 

fig2_tree

Figure 2. Phylogenetic tree of snakes species in the families Colubridae and Viperidae, showing how tail-rattling behavior has evolved.
As shown in Allf et al. (2016)

Now follow the lines (branches) next to each species, from right to left. Every time two branches come together you’ve reached a node, which represents the common ancestor of those two species. A pie chart on each node shows the probability of that ancestor possessing the trait of tail vibration. You should quickly notice that almost every ancestor has a completely white pie (a single black line in it as opposed to a black wedge) indicating a nearly 100% chance that that ancestor vibrated its tail. These likelihood values provide statistical support for what already seemed obvious, that the behavior of tail vibration evolved long before the physical rattle of rattlesnakes.

But there’s more to this study than simply confirming that rattling evolved before rattles. Seeing how a particular morphological trait functions often makes perfect sense at first glance: wings are great for flying, eyes great for seeing, teeth great for biting, etc. But how do these highly specialized structures arise in the first place? In other words, how does evolutionary novelty come about?

Evolution is NOT Random

There is a common misconception that evolution is completely random, causing many people to be skeptical of how such a wide variety of well-functioning organisms could have evolved on Earth without some sort of divine intervention. You wouldn’t expect to build a computer by randomly throwing circuit boards and wires at each other, right? So how could evolution produce such a huge variety of organisms so perfectly suited to their environments, with no predetermined design or direction? It’s because the view that evolution is 100% random is wrong, there’s more to it than that.

Endless forms

It’s difficult for some people to imagine how such a variety of plant, animal, and microbial forms of life have evolved. Behavior-facilitated adaptation is just one way that evolutionary novelty can come about.
Illustration by Ainsley Seago (www.trufflebeetles.com)

We do know that genetic variation—a crucial component of evolution—arises completely at random (for the most part) by the process of mutation. Most mutations are random mistakes made by cells as they make copies of their DNA before dividing, and the vast majority of mutations have effects that are usually either negative or neutral. It’s very rare for a random mutation to actually improve the survival or reproductive abilities of the organism who gets it.

But another critical piece of the evolutionary process is natural selection, which is anything but random. Natural selection is extremely effective at teasing out even the most subtle advantages among individuals. This means that if a trait has any influence (no matter how small) on increasing the survival and reproductive chances of some individuals relative to others, then natural selection is said to favor that trait. This causes a beneficial mutation to become more and more common each generation, until it belongs to every member of the species. Natural selection is so good at doing this that it can favor changes that are imperceptible to even the most observant scientists with the most expensive instruments.

Can Environment Induce New Adaptations?

Despite the extreme non-randomness of natural selection, it can still only work with what it’s given. In other words, while the environment is incredibly effective at causing species to adapt over time by favoring individuals with the best traits, it can do nothing to make those traits appear in the first place. Or can it?

Well, the rattlesnake study says…sorta maybe. This is not to say the environment directly induces beneficial DNA mutations in an organism (except epigenetics now tells us that it sort of can), but it does appear that tail-rattling behavior provided the perfect backdrop for evolution of the rattle itself. The authors suggest that this might have happened in one of two ways.

The first hypothesis is that tail-rattling behavior as a warning signal simply provided a situation that allowed a rattle to evolve. Let’s say genetic variation among individuals who all rattled their tail meant that some snakes produced a little more keratin on the tips of their tails than others. This could cause these individuals to make a bit more noise when they vibrated their tails, making them a bit better at startling predators. These individuals would survive longer and leave more offspring, some of which would produce even a bit more keratin tissue on the tail than their parents. Continued selection for increased keratin could eventually lead to the fully formed rattle as it exists today. If tail vibration hadn’t existed first, then selection never would have favored individuals who happened to grow a bit more tail keratin.

Alternatively (and more remarkably IMO), rattling behavior may have actively facilitated rapid evolution of the rattle. Like, actually caused the rattle to form. In this scenario, tail vibrations is hypothesized to have caused a sort of callus to form on the tip of the tail. Snakes that vibrated their tails more rapidly and for a longer duration would have developed larger calluses. If certain mutations made some snakes more susceptible to callus formation than others, and if these individuals were also better at deterring predators, then selection would favor callus formation associated with tail vibration.

This would mean that the environment helped cause the formation of the rattle as opposed to simply favoring random mutations that gradually made the rattle bigger. It may sound like a subtle distinction, but in evolutionary biology it’s huge.

While the researchers are not yet able to say which scenario is more likely, more results are consistent with the latter hypothesis. When they mapped two aspects of tail vibration onto the snake phylogeny, they found that duration was longest and intensity was highest in rattlesnakes and their close ancestors:

Allf 2016 7

From Allf et al. (2016)

But if tail calluses only developed when snakes behaved a certain way (i.e. rattled their tails), then we wouldn’t expect them to pass those calluses on to their offspring. That would be similar to your kids inheriting the big muscles you earn by hitting the gym every day. This type of evolution where an individual acquires a trait during its lifetime and passes it to offspring is called Lamarckian inheritance, and it’s something evolutionary biologists disproved decades ago (although read more on epigenetic Lamarckism if you really want to fall down a rabbit hole).

giraffes-are-heartless-creatures-Jan-2012-tiny

A classic example of Lamarckism is the giraffe acquiring a longer neck during its lifetime and passing on that trait to its offspring. We now know evolution doesn’t work this way (mostly).

Of course reality is rarely as simple as we’d like it to be. While natural selection may not have directly induced mutations that grow rattles, it could have provided a strong advantage to snakes that rattled their way to callused tails. And if this selection pressure remained strong for a long period of time, the stage was set to favor any mutation that may have caused even a tiny bit of extra tail keratin formation, independent of the behavior itself. We’d call this genetic assimilation, where a trait that originally forms in response to environmental stimulus eventually becomes hard-wired by the genome. In other words, formation of rattles (the noun) became programmed into the development of rattlesnakes even though their ancestors had to work hard for it, by rattling (the verb).

It may seem counterintuitive that the use of a tool could evolve before the tool itself, but future studies may show that’s it far more common in nature than we now realize. While a resurgence in neo-Lamarckism isn’t likely, it is possible that behavior can be an effective precursor, and even facilitator, of the evolution of novel structures.


*The “king” in kingsnake references the fact they often eat other snakes, including rattlesnakes – they’re resistant to rattlesnake venom (which is obviously badass).

1Bradley C. Allf, Paul A. P. Durst, and David W. Pfennig,“Behavioral Plasticity and the Origins of Novelty: The Evolution of the Rattlesnake Rattle,” The American Naturalist (Volume & page numbers not yet available) DOI: 10.1086/688017

Ants can medicate themselves when they’re sick

Whether swallowing Excedrin® for a headache or sipping a cocktail to take the edge off, you’ve likely self-medicated more than once in your life. Similar to written language and advanced tool use, ingesting an otherwise harmful substance for the purposes of curing what ails you is a behavior usually considered to be uniquely human. But it turns out ants can self-medicate, too. A recent study showed that common black ants changed their diets when they were sick—eating more food supplemented with a medicinal substance—as compared to ants with no infection.

1024px-Grauschwarze_Sklavenameise_Formica_fusca_02_(MK)

The common European black ant, Formica fusca, can alter its behavior to self-medicate when it’s sick.
Photo by Mathias Krumbholz via Wikimedia Commons

Do animals get high?

You may have heard stories of elephants getting drunk by eating fermented fruit or of dolphins getting high off pufferfish toxin – but as much as people might like these stories to be true, they’re actually driven more by myth than by the reality of evolution.

It shouldn’t be surprising that natural selection would not favor a tendency to get high. Being successful in a predator-prey interaction, for example, requires being substantially more alert than a stoner on their couch eating pizza rolls and watching Netflix.

stoner-on-the-couch

Intentional inebriation may enhance the movie-watching experience for Harold and Kumar in their living room, but it’s not a particularly good evolutionary strategy.
Image copyright of Warner Bros.

But what about legitimate medicine? Disease is pervasive in the natural world. If a creature could cure itself of infection by ingesting a substance naturally available to them—and if this behavior was genetically inherited—then we could easily expect the trait to spread through a population over time (in other words, we’d expect the species to evolve the ability to self-medicate).

A study by researchers at University of Helsinki in Finland (published online for an upcoming issue of the journal Evolution) has demonstrated this sort of therapuetic self-medication in the common European black ant, Formica fusca. They found that the ants will intentionally ingest hydrogen peroxide (H2O2) in the presence of a fungal pathogen, and that the otherwise toxic substance helps them recover from the infection.

Specific criteria for self-medication

To show that this behavior is genuinely medicinal, some specific criteria needed to be met. If the ants ate a non-toxic substance that just happened to prevent infection, for example, this could potentially be eaten by all ants as part of their diet—regardless of pathogen exposure—and would not qualify as medicine. In order to be considered self-medication, the ingested substance must: (1) increase the fitness of infected ants (i.e., make it healthier), (2) have a negative effect on uninfected ants, (3) have a negative effect on one or more pathogens, and (4) be deliberately consumed by the ants.

customized_ants_wine_holders

Two metal ants holding a bottle of wine and looking happily medicated.
Image from http://www.frbiz.com, a website that sells things.

In a cleverly designed study, researchers found that their study subjects fulfilled all four of these criteria. First, they divided ants into two colonies, one of which was fed a standard honey-based food while the other colony was fed the same food, but supplemented with hydrogen peroxide (they use the more general term reactive oxygen species, or ROS, in the paper). Each of these groups was then divided into two, one exposed to the common fungal pathogen, Beauveria bassiana, and the other kept as a fungus-free control group. They tracked daily mortality rates of all four groups for 12 days and found that medicated ants survived at a higher rate than non-medicated when exposed to the fungus, but the opposite was true for the pathogen-free control groups. This satisfied criteria 1 and 2.

Bos 2015 fig1

Graph showing that ingesting H2O2 (dotted lines) increased the survival of fungal-infected ants (black arrow) while decreasing survival in healthy ants (gray arrow).
Figure 1 from Bos et al. (2015)

To demonstrate that these differential survival rates were the direct result of H2O2 killing the fungal infection, they cultured the fungus in petri dishes with six different concentrations of H2O2, including 0%. This part of the experiment showed a steep decline in the survival of the fungus as the concentration of H2O2 increased, satisfying criterion 3.

Bos 2015 fig2

Graph showing the rapid decline in the survival of the fungal pathogen BB in the presence of increasing concentration of H2O2.
Figure 2 from Bos et al. (2015)

Finally, for the most interesting part, the authors tested whether ants would deliberately put themselves into contact with H2O2 after being infected with the fungus. If they did, this would demonstrate that the ants are able to distinguish between medicinal and non-medicinal food, and that they only eat the medicine when they’re sick. You probably know where this is going: when given a choice, infected ants indeed increased their consumption of H2O2-supplemented food, while non-infected ants largely avoided it.

Bos 2015 fig3

Graph showing that, when given a choice, up to 50% of fungal-infected ants deliberately consumed food supplemented with H2O2, while no more than ~20% of non-infected ants were ever observed doing the same. The results also suggest ants adjust the amount of medicine they take – consuming more of it at 4% than at 6% concentration.
Figure 3 from Bos et al. (2015)

It was a very simple and straightforward experiment, but one that carefully demonstrated all the criterion necessary to say that these ants successfully self-medicate based on the presence of infection. While this work was done in the lab, the authors are careful to point out that ants are also likely to encounter reactive oxygen species such as H2O2 in nature, as they can be present in many ant food sources such as nectar and cadavers.

Free meds

The authors even speculate that the phenomenon may be part of a mutualistic association between aphids and ants. Aphids secrete a substance called honey dew as they feed on plants, and ants are known to feed on the honey dew. Because plants also produce H2O2 as a defense against aphid predation, infected ants could potentially use the honey dew food source as a way to adjust their intake of H2O2.

Also posted at: http://www.planetexperts.com/self-medicating-ants/

Nick Bos, Liselotte Sundström, Siiri Fuchs, and Dalial Freitak. 2015. Ants medicate to fight disease. EvolutionDOI: 10.1111/evo.12752

Omega-3 fatty acids are probably good for your heart…if you’re descended from Inuits

You’ve probably heard that omega-3 fatty acids—abundant in fish, nuts, and hippie favorites like flax and chia seed—are good for your heart. For decades we’ve been told that they help reduce the risk of heart attack and stroke. It’s estimated that about 10% of Americans take fish oil supplements primarily for this reason. But now a critical look at this claim through the lens of evolutionary biology suggests most of these people may be wasting their time and money if they’re hoping those shiny pills will give them a healthy heart.

It's common knowledge that a diet high in omega-3 fatty acids is good for your heart – except it's probably not, unless you're Inuit.

It’s common knowledge that a diet high in omega-3 fatty acids is good for your heart – except it’s probably not, unless you’re Inuit.

The origins of the supposed health benefits of omega-3s began in the 1970s when Danish researchers were studying Inuit metabolism.1 Because vegetation can’t grow at such high latitudes, the Inuit diet has historically consisted almost entirely of meat from fish, seals, whales, and other Arctic animals.

All of these meats are high in fat, of course, yet Inuit people rarely suffer from the heart attacks that are so common among Westerners with similarly high-fat, highly carnivorous diets. The researchers concluded that this is due to high levels of omega-3 polyunsaturated fatty acids found in fish serving as protection from heart disease. Doctors have recommended diets high in fish and/or fish oil supplements for a healthy heart ever since.

Inuit people and their ancestors have subsisted on a diet of fatty arctic fish and meat—like this unfortunate walrus—for centuries.

Inuit people and their Beringian ancestors historically subsisted on a diet of fatty arctic fish and meats—like this unfortunate walrus—for centuries.

But there’s one little problem with this bit of conventional wisdom – subsequent studies have failed to provide evidence that fish oil actually reduces heart disease, and now researchers at UC Berkeley and the University of Greenland say they’ve figured out why (see their results here, published this week in Science).

They scanned the genomes of Greenlanders, and compared the ones with more than 95% Inuit ancestry with those of more European descent. This led them to discover some gene variants that were unusually common in the Inuit bloodlines. Many of these variants occurred in a cluster of genes called FADS, so named because they control the production of enzymes called fatty acid desaturases. FADS are known to regulate fat levels in the body, including omega-3 fatty acids.

One particular gene variant in the FADS cluster was present in almost every Inuit they studied, compared to only a quarter of Chinese people and just 2% of Europeans. Here’s the upshot: while omega-3 fatty acids may very well have been protecting Inuits from heart attacks despite their highly carnivorous diets, it doesn’t do the rest of us much good because we just don’t have the proper genes that allow our physiology to make it happen.

Why not? Because evolution by natural selection happens to humans just like any other living thing. In this case it worked on Inuits ~20,000 years ago when they inhabited Beringia, the region where Alaska and Russia came together at the time because sea levels were much lower.

Low sea levels during the most recent glacial maximum allowed Inuit ancestors to live on land in the Beringia region. Image from Wikipedia Commons.

Low sea levels during the most recent glacial maximum allowed Inuit ancestors to live on land connecting Russia to Alaska in the region called Beringia (outlined in green).
Image from Wikimedia Commons.

The genetic mechanism behind this is very similar to another one that you might be uncomfortably familiar with: lactose intolerance. The ability of mammals to digest lactose—the primary sugar found in milk—typically disappears after weaning because their bodies stop producing lactase, the enzyme that breaks down lactose. But in the human genome, three different mutations set us apart from other mammals, each allowing our bodies to continue producing lactase into adulthood. If you’re lactose intolerant, you sadly missed out on inheriting one of these mutations.

If you can metabolize lactose as an adult, you have at least one little mutation in your genome to thank.

If you can metabolize lactose as an adult, you have at least one mutation in your genome to thank.

What’s really interesting is that at least one of these three mutations is very common in European, African, and Middle Eastern populations that historically domesticated cattle, but far less common in non-pastoralist groups from Africa and Asia. In other words, evolution happened to humans when natural selection favored mutations allowing adults to metabolize a rich source of protein that was previously unavailable to them. This is just one of many known examples of human adaptations to their environment.

Illustration by A. Cuadra (Science Magazine) and Meagan Rubel (Univ. of Pennsylvania) showing genetic adaptations to diverse environments during human evolution. Image from Strength in small numbers by Sarah Tishkoff, Science Vol. 349:1282-1283.

Illustration by A. Cuadra (Science Magazine) and Meagan Rubel (Univ. of Pennsylvania) showing genetic adaptations to diverse environments that have arisen during the course of human evolution.
Image from “Strength in Small Numbers” by Sarah Tishkoff, Science Vol. 349:1282-1283.

So the FADS gene variant that’s uniquely common in the Inuit genome helps them desaturate the high levels of omega-3 fatty acids in their diets. Saturated fats clog arteries more readily than unsaturated fats, so this gene effectively protects Inuits from heart disease despite a high-fat diet in a way that most Europeans are unable to take advantage of.

This Inuit family more than likely possess a gene variant that allows omega-3 fatty acids to protect them from heart disease. Most people in the rest of the world don't.

There’s about a 95% chance this Inuit family possessed a gene variant that modified omega-3 fatty acids in a way that protected them from heart disease. Most people in the rest of the world don’t have this ability.
Photo from National Geographic Magazine, 1917.

This illustrates an important point about how we understand the physiology of our own bodies. As much as marketers would like you to believe they have a food or supplement that acts as a magic bullet for your health, they’re don’t. Our genes play a huge role in how our bodies react to the foods we eat and the drugs we take. As we continue to better understand the functional complexities of the human genome, more variations between populations and individual people will reveal themselves.

Some foods are certainly more healthy than others, but if health claims seem too magical to be true, they almost certainly are.

Some foods are certainly more healthy than others, but beware of superlatives.
Image from bokusuperfood.com

There’s also a lesson to be learned here about the scientific process in general. You’re certainly familiar with seeing news stories about a study that says something is good for you—coffee, red wine, chocolate, etc.—only to have another study contradict those results a few years or even months later.

This can sometimes leave people with the impression that scientists don’t really know what they’re doing, but this is far from the truth. In almost all cases this problem arises as a result of media coverage that overstates the implications of a study for the sake of ratings.

Despite the allure of huge discoveries, the scientific process typically moves at a more gradual pace. Scientists learn about the natural world in bits and pieces as they accumulate data over time. To be sure, some steps of this process are bigger than others, and some scientific interpretations are eventually contradicted by new evidence. But because science inherently corrects itself, it’s incredibly efficient at marching forward.

In the case of omega-3 fatty acids, the 1970s Danish research was correct in concluding that they protect Inuits from heart disease. The problem is that it was not yet known that genetics made this particular group of people unique in that respect, so doctors and scientists were incorrect in assuming the health benefits would apply for all humans.

One of the most prominent evolutionary biologists of the 20th century, Theodosius Dobzhansky, once wrote an essay titled “Nothing in Biology Makes Sense Except in the Light of Evolution.” The recent Inuit study is a perfect example of this. It illustrates not only that evolution happens, but also how understanding how it happens can be really important to our daily lives.

1Info about the 1970s research came from Carl Zimmer, who actually makes a living writing about science, in this NY Times piece he wrote about the study.

New species of ancient human discovered in South African cave (maybe)

Scientists in South Africa have found the remains of another one of our ancient cousins, and the fossil collection they used to describe the new species is exceptionally large and complete.

Modern humans (Homo sapiens) constitute the only living species in the genus Homo, but several species coexisted tens to hundreds of thousands of years ago. Now a previously undiscovered species of ancient human (Homo naledi) has been described using the largest collection of bones (1550+) from a hominin (the group of apes that includes humans and chimps) ever found in Africa. The collection is made up of fossils from at least 15 individuals, and nearly every bone in the skeleton is represented multiple times – likely a result of the species burying their dead at the site over an extended period of time.

Figure 1 from Berger et al. (2015) shows nearly all of the skeletal elements used to describe Homo naledi.

Nearly all of the skeletal elements used to describe Homo naledi—more than 1,500 bones in all—displayed on a 4-foot wide table.
Image from Fig. 1 of Berger et al. (2015)

The species is named after the Rising Star cave system of South Africa, where the bones were found. The lower legs, feet, and hands are very similar to yours and mine. The ribcage, shoulders, pelvis, and small braincase, however, are more similar to earlier species of Homo that lived 2–4 million years ago. This combination of features suggests that an enlarged brain was not necessary in order to develop human-like articulation of hands and feet.

Every bone of the modern human-like hands except for a small wrist bone called the pisiform was recovered. Image from Fig. 6 of Berger et al. (2015)

Every bone of the modern human-like hands except for a small wrist bone called the pisiform was recovered.
Image from Fig. 6 of Berger et al. (2015)

So what else does this say about the evolution of modern humans? Analysis of the fossil collection is still in its early stages, so much work remains to be done. For one thing, these fossils have not yet been dated so we don’t know their exact age. It’s also difficult to accurately place them into the evolutionary tree of humans, in part because fossil collections of most other ancient humans are far less complete than this one. While this work pieced together a nearly complete skeleton of H. naledi, most other species of ancient humans have been described using far less evidence – sometimes as little as a single fragment of a bone.

What we can be certain of is that H. naledi shares a fairly recent common ancestor with H. sapiens, but that does not necessarily suggest that we descended from them directly. In other words, there is no evidence so far that H. naledi “evolved into” modern humans. This may seem counterintuitive, which brings up an important point about evolution that is often misunderstood (and so now I’ll digress for a bit).

You’re probably familiar with the iconic image below, often used to illustrate the evolution of humans – an ape drags its knuckles on the ground, then walks on two legs with shoulders hunched forward, gradually straightens its posture to resemble what we’d call a caveman, and eventually stands upright to look just like us. It makes for a great piece of art and has been the subject of many jokes, but unfortunately it’s an almost entirely inaccurate representation of how evolution happens.

evolitio

This image does not illustrate the process of evolution, despite the fact that so many people use it to do so.
Image from http://www.frontrangeforum.org

The problem with this illustration of evolution (human or otherwise) is that it appears to happen in a straight line, but evolution is really a branching process full of splits and stops. Imagine following a species through time, but instead of a species let’s call it a lineage. It is certainly true that a lineage can change over time to be different than it was in the past, but what really generates the diversity of life we see today is the splitting of these lineages.

This is what Charles Darwin articulated in On the Origin of Species, and the only figure he included in that book is a drawing that shows how this happens. One lineage may split into two or more lineages, it may remain a single lineage for a while, or it may go extinct. What is certain is the change through time (evolution is always happening), which is why it shouldn’t be surprising that we evolved from an ape-like ancestor even though apes still exist. This is a favorite “gotcha” question for those wanting to deny evolution, but it makes absolutely no sense. What no longer exists is the single ancestral lineage that split and gave rise to both us and modern apes. That ancestor only exists in the past.

The only figure in Darwin's On the Origin of Species shows how lineages can change, split, and go extinct through time.

The only figure published in Darwin’s On the Origin of Species shows how lineages can change, split, and go extinct through time.

Lineages that don’t split but instead go extinct are often referred to as evolutionary “dead ends.” While the collection of bones representing H. naledi has not yet been examined in enough detail to determine its ultimate fate, it’s likely that this particular hominin lineage eventually became one of those dead ends. In fact, as modern Homo sapiens spread across the globe they outcompeted all other species of ancient humans, driving them to extinction.

But rampant extinction isn’t unique to humans. The reason evolution seems to have worked so well in shaping organisms for their respective environments is that it fails constantly, but what we see around us are the relatively few winners because they’ve survived. This can give the impression that species are somehow “designed” for their environment, when they really are just the small minority of random evolutionary innovations that actually worked.

Anyway, back to story of our new cousin. With this discovery in South Africa, Homo naledi becomes the eighth well-established species in the genus Homo. We have a better understanding of the evolutionary history of humans than probably any other group of organisms – partly because of how recently it has all gone down, but also because we tend to put a lot of effort into studying ourselves. Despite this, there is still much to learn about our past, and discoveries like this one will continue to add fascinating details to the story.

We’ll never know everything about how life evolved on Earth, but that’s what makes it such a fascinating subject to study. There will always be more fossils to find and missing pieces of the story to fill in…

UPDATE: As is the case with many scientific discoveries—particularly those that get a lot of media attention—this one is not without controversy. It turns out that at least a few peers of the researchers behind this study disagree that the evidence gathered so far supports the status of Homo naledi as a new species. A couple of quotes from the objectors:

“From what is presented here, they belong to a primitive Homo erectus, a species named in the 1800s.” – Tim White, paleoanthropologist University of California, Berkeley

“Intentional disposal of rotting corpses by fellow pinheads makes a nice headline, but seems like a stretch to me.” – William Jungers, anthropologist at Stony Brook School of Medicine

“The ‘new species’ and ‘dump-the-dead’ claims are clearly for the media. None of them is substantiated by the data presented in the publications.” – Christoph Zollikofer, anthropologist at the University of Zurich

Ouch. As I mentioned, there is still plenty left to analyze in this collection and many more bones remain buried in the cave. It’s entirely possible that additional fossils and/or analyses of those already excavated will sway the opinions of these detractors. Of course that’s not a given and only time will tell. You can read more about the original claims and the criticism they’ve received here, in The Guardian.

Despite its reputation for objectivity, science is a fluid and gradual process, and evidence is always open to interpretation. The only way to reach consensus is to gather new data, which is certainly what will happen in this case. It’s a good lesson to be wary of headlines you see in the popular media (and blog posts), as anything that claims to be a groundbreaking study is likely either overstating their conclusions and/or ignoring some aspect of the topic that is already known.

So while we can be confident that science will continue to march forward and ever closer to certain truths in the long run, we can also count on some ups and downs along the way. Keep your eyes open to find out how old this huge pile of bones is, whether it really does represent a species distinct from Homo erectus, and whether that species actively buried their dead.

Works cited:

Berger et al. 2015. eLife 2015;4:e09560. doi: http://dx.doi.org/10.7554/eLife.09560#sthash.nsItjZZ1.dpuf

Dirks et al. 2015. eLife 2015;4:e09561. doi: http://dx.doi.org/10.7554/eLife.09561#sthash.MH9nYsNY.dpuf

Sea slugs in Bocas del Toro, Panama

UPDATE: If you happen to be interested in more scientific details, you can download a paper detailing all the species we found here.

I recently returned from Panama where I helped teach a workshop on the taxonomy & biology of sea slugs at the Smithsonian Tropical Research Institute’s Bocas del Toro research station. Getting back into the ocean after being stuck at a computer desk or lab bench for 3 years inspired me to bring this blog out of hibernation.

Students from 7 countries gathered at the STRI research station in Bocas del Toro, Panama for a 2-week workshop on the taxonomy and biology of sea slugs.

Students from 7 countries gathered at the STRI research station in Bocas del Toro, Panama for a 2-week workshop on the taxonomy and biology of sea slugs.

The workshop was organized by my post-doc advisor at Cal State LA, Patrick Krug, and Cal Poly Pomona professor Ángel Valdés. Participants included 5 students from these two labs in Southern California, plus enthusiastic undergraduate and grad students from Florida, Maryland, Costa Rica, El Salvador, Mexico, Colombia, India, and Brazil – only 13 students but they represented 7 different countries!

Lectures and lab activities relating to the taxonomy and evolution of sea slugs were necessary, but we also spent many hours snorkeling in the shallow waters around Isla Colón, an island off the Caribbean coast of Panama. The island belongs to the Bocas del Toro Province, which borders Costa Rica.

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Twice a day for two weeks we went out on a boat to snorkel for sea slugs. Here we are looking extremely safe in our life jackets during one of the few moments the sun came out.

The best way to learn about sea slugs and most other animals is to observe them, so we needed to collect as many as possible – and the numbers suggest we did a decent job. Only 19 different species of sea slugs had been officially documented from the waters surrounding Isla Colón (Collin et al. 2005; pages 689-693). By the time we finished 12 days of collection and observation, we had catalogued nearly 80 distinct species. More than 20 of these have never been formally described, meaning they are entirely new to science! The class is currently working to put together a field guide-type manuscript to list all of the species we found. Much more work is needed to verify and formally describe what we suspect are new species.

The brilliantly colored aeolid Cuthona caerulea, along with some of its eggs.
Photo by Ángel Valdés

Sea slugs are closely related to their more familiar cousins, terrestrial and freshwater snails and slugs. The evolutionary history among all these groups has not yet been fully resolved, but DNA sequence evidence points to an interesting story. The most comprehensive genetic study to date (Kocot et al. 2013) suggests that one lineage of sea snails developed drastically reduced shells (or lost them altogether) in becoming the ancestor of all sea slugs. Then as this ancestor diversified into many species of sea slugs, one of their descendant lineages slowly made the transition to land, evolved a lung, and became the common ancestor of all terrestrial (and freshwater) snails and slugs. DNA sequences from additional genes and/or species could either contradict or further support this interpretation.

Sea slugs display an incredible diversity of color and form. Photos by Ángel Valdés.

Sea slugs display an incredible diversity of color and form.
Photos by Ángel Valdés

Sea slugs themselves are divided into four main groups. The most well-known and biodiverse group is Nudibranchia – these carnivores feed on sponges, cnidarians (jellyfishes, corals, hydroids, etc.), and even other sea slugs. Lacking the protective shells of their snail cousins, most sea slugs use nasty toxins for defense, which they either synthesize or harvest from their prey. This is what facilitates the evolution of their brilliant aposematic coloration.

Berghia rissodominguezi

Two individual slugs of the aeolid species Berghia rissodominguezi crawling on rock.
Photo by Gina Lopez

Along with or as an alternative to chemical toxins, many species of aeolid nudibranchs also possess the remarkable ability to steal nematocysts (stinging cells) from their cnidarian prey without setting off the stinger – the firing of these cells is what causes the pain of a jellyfish sting and the stickiness you feel when you touch an anemone. The slugs store these stinging cells in finger-like projections called cerata, using them for defense against other predators.

Sometimes it seems like the most beautiful slugs don't look like animals at all – Tritonia bayeri. Photo by Ángel Valdés.

Sometimes it seems like the most beautiful slugs don’t look like animals at all – here’s the aeolid Tritonia bayeri.
Photo by Ángel Valdés

Dondice parguerensis is seen here in its natural habitat, living and feeding on the upside down jellyfish Cassiopea xamachana. Photo by Ángel Valdés.

Dondice parguerensis is seen here in its natural habitat, feeding on and receiving protection from the tentacles of an upside down jellyfish, Cassiopea xamachana
Photo by Ángel Valdés

The bright colors of Dondice occidentalis likely evolved as a warning to predators of the stinging nematocysts stored in its cerata. Photo by Ángel Valdés.

The bright colors of Dondice occidentalis likely evolved as a warning to predators of the stinging nematocysts stored in its cerata.
Photo by Ángel Valdés

Another group of nudibranchs, the dorids, feeds primarily on sponges and lacks the cerata that are distinctive of aeolids. The “sea bunny” whose photos recently went viral is an example of a dorid. Dorids have an exposed but retractable cluster of gills near their posterior called a branchial plume. This organ is actually what gives rise to the name nudibranch, which is derived from the Latin word for naked and Greek word for gill. Most nudibranchs are relatively small—just a few inches long and in some cases shorter than a centimeter—but one dorid in the Pacific Ocean, the Spanish dancer Hexabranchus sanguineus, can reach lengths of over 40 cm! Click here to see its Caribbean sister species (H. morsomus) display its flappy swimming technique when disturbed.

Hexabranchus morsomus, one of the largest nudibranchs we found. Photo by Ángel Valdés.

Hexabranchus morsomus, the largest nudibranch we found in Bocas.
Photo by Ángel Valdés

Two different individuals of the same species, Dendrodoris krebsii. As you can see, it's one of the most variably colored nudibranchs in the world. Photos by Ángel Valdés.

Two different individuals of the same species, Dendrodoris krebsii. As you can see, it’s one of the most variably colored nudibranchs in the world.
Photos by Ángel Valdés

While snorkeling I noticed a bright orange spiraled egg mass on a rock, then found these two Platydoris angustipes underneath. Photo by Ryan Ellingson.

While snorkeling I noticed a spiraled bright orange egg mass on a rock, then found these two Platydoris angustipes underneath.
Photo by Ryan Ellingson

Some dorids look more like an artist's creation than an animal – Doriprismatica sedna. Photo by Ángel Valdés.

Some dorids look more like an artist’s creation than an animal – Doriprismatica sedna.
Photo by Ángel Valdés

A second group of carnivorous sea slugs is called Cephalaspidea – the headshield slugs and bubble snails. These slugs have a characteristic head shield that is used tosense their surroundings and burrow through the upper surface of sand. Most species have retained the shell of their snail ancestors, though it is dramatically reduced (small and/or thin) and often internal so it fails to provide any meaningful protection. They feed primarily on other sea slugs and various types of marine worms, often swallowing their prey whole and crushing it in a gizzard that contains calcareous plates. They are such voracious predators that some have even been observed cannibalizing other members of the same species.

Chelidonura berolina

The cephalaspidean Chelidonura berolina.
Photo by Ángel Valdés

Haminoea elegans is part of a small group commonly called bubble snails because of their delicate shells, but they are more closely related to slugs than snails. Photo by

Haminoea elegans is part of a small group commonly called bubble snails because of their delicate shells, but they are more closely related to slugs than snails.
Photo by Ángel Valdés

A third group of slugs is called Aplysiomorpha (or Anaspidea, depending on who you ask), commonly known as sea hares. Sensory structures on the head called rhinophores are a characteristic of nearly all sea slugs, except for cephalaspideans. The name sea hare comes from the large, rolled rhinophores that somewhat resemble rabbit ears. This group contains the largest known slug, the black sea hare Aplysia vaccaria. All sea slugs are hermaphrodites, and sea hares in particular are known to form daisy chain orgies when mating – you’re welcome.

It may just look dirty, but Bursatella leachi is actually covered in fleshy papillae that help it blend in with its surroundings. Photo by Ángel Valdés.

It may just look dirty, but Bursatella leachi is actually covered in fleshy papillae that help it blend in with its surroundings.
Photo by Ángel Valdés

Another sea hare, Dolabrifera dolabrifera. Photo by Ángel Valdés.

Another sea hare, Dolabrifera dolabrifera.
Photo by Ángel Valdés

While many slugs are brightly colored to warn predators of their toxicity, others are incredibly cryptic. Can you spot Phyllaplysia engeli here? Photo by Ángel Valdés.

While many slugs are brightly colored to warn predators of their toxicity, others are incredibly cryptic. Here’s an example of Phyllaplysia engeli blending into its surroundings.
Photo by Ángel Valdés

The spotted sea hare Aplysia dactylomela is found in tropical and temperate waters around the world. As with many cosmopolitan species, genetic studies may reveal that what we currently recognize as a single species may be more than one. Photo by Sabrina Medrano.

The spotted sea hare Aplysia dactylomela is found in tropical and temperate waters around the world. As with many cosmopolitan species, genetic studies may reveal that what we currently recognize as a single species may be more than one.
Photo by Sabrina Medrano.

The last and arguably most interesting group of slugs is Sacoglossa – the group that happens to be the focus of my current research. Sacoglossans eat algae and are sometimes referred to as sap-sucking sea slugs. This common name comes from the fact that many species feed by poking a hole in an algal filament and sucking out the insides. This works because the algae is siphonaceous; a single filament contains many cells but no cell walls that divide them. This means a slug can use its tooth (called a radula) to puncture an algal filament and then suck out all the nutritious cells. Under a low-powered microscope you can actually watch a slug wrap its mouth around a filament and drain it of its green cells, leaving it empty and transparent.

Elysia crispata is one of the largest and most morphologically variable sacoglossans, and probably the most familiar to divers and hobbyists. Photos by Ángel Valdés.

Elysia crispata is one of the largest and most morphologically variable sacoglossans, and probably the most familiar to divers and hobbyists.
Photos by Ángel Valdés

Not to be outdone by their nematocyst-thieving cousins, many sacoglossans practice kleptoplasty – they steal photosynthetic chloroplasts from their algal food source and store the organelles in their own tissues. These chloroplasts remain functional as slugs use the sun to generate sugars via photosynthesis, allowing them to go weeks or even months without eating food. There is even strong evidence that at least one species has taken photosynthesis genes from algae and incorporated them into its own genome – a remarkable adaptation that allows an animal to make its own food from sunlight, water, and carbon dioxide just like a plant.

Elysia ornata crawling on some filaments of Bryopsis green algae, on which it feeds. Photo by Ángel Valdés.

Elysia ornata crawling on some filaments of Bryopsis green algae, on which it feeds.
Photo by Ángel Valdés

It was great to meet students from so many different countries and work together to document many more species of sea slugs than had ever been catalogued from the waters around Bocas del Toro. Even though we primarily hunted for sea slugs, it’s hard not to marvel at all the other marine life you come across when spending so much time in tropical reef habitats. Below are some pics worth seeing even though they’re not slugs:

Two flamingo tongue snails (Cyphoma gibbosum) on a gorgonian coral.
Photo by Ryan Ellingson

Sea horse (Hippocampus erectus?) found among mangrove roots. Photo by Gina Lopez.

Seahorse (Hippocampus erectus?) found among mangrove roots.
Photo by Gina Lopez

The sea cucumber Ocnus suspectus inserts its tentacles into its mouth one at a time, scraping off bits of food as it pulls the tentacle back out. Click here to see this behavior in action. Photo & video by Ryan Ellingson.

The sea cucumber Ocnus suspectus inserts its tentacles into its mouth one at a time, scraping off bits of food as it pulls each tentacle back out. Click here to see this behavior in action.
Photo and video by Ryan Ellingson

Sea anemone Condylactis gigantea(?). Photo by Ryan Ellingson.

Sea anemone Condylactis gigantea(?).
Photo by Ryan Ellingson

The brittle star Ophiothrix suensonii is highly variable in color, wraps itself around many different species of sponge, and is very common in Bocas.

The brittle star Ophiothrix suensonii is highly variable in color, wraps itself around many different species of sponge, and is very common in Bocas.
Photo by Ryan Ellingson

The caribbean reef octopus Octopus briareus. Click here to see it change shape and color as it retreats from me into mangrove roots. Photo and video by Ryan Ellingson.

The caribbean reef octopus Octopus briareus. Click here to see it change shape and color as it retreats from me into mangrove roots.
Photo and video by Ryan Ellingson.

Ascidians (this one is Polycarpa spongiabilis) just hang out and filter feed like sponges, but are very far away on the tree of life – despite their apparently simple morphology they are actually more closely related to vertebrates than any other marine invertebrate. Photo by Ryan Ellingson.

Ascidians just hang out and filter feed like sponges, but are very far away on the tree of life – despite their apparently simple morphology they are actually more closely related to vertebrates than any other marine invertebrate. This species is Polycarpa spongiabilis
Photo by Ryan Ellingson

The spotted cleaner shrimp Periclimenes yucatanicus lives among the tentacles of anemones. It's a type of cleaner shrimp that waves its tentacles to attract fish so that it can eat algae, parasites and dead tissue from the fish's body. Photo by Ryan Ellingson.

The spotted cleaner shrimp Periclimenes yucatanicus lives among anemone tentacles. It’s a type of cleaner shrimp that waves its antennae to attract fish so that it can eat algae, parasites and dead tissue from the fish’s body.
Photo by Ryan Ellingson

The urchin Lytechinus variegatus uses its tube feet to grab onto and cover itself with pieces of sea grass, shells, coral rubble, and other debris.

The urchin Lytechinus variegatus uses its tube feet to cover itself with pieces of sea grass, shells, coral rubble, and other debris.
Photos by Ryan Ellingson

The Magnificent Feather Duster worm Sabellastarte magnifica. Photo by Ryan Ellingson.

The Magnificent Feather Duster worm Sabellastarte magnifica.
Photo by Ryan Ellingson

The resident caiman at the STRI research station in Bocas. Photo by Ryan Ellingson.

The resident caiman at STRI research station in Bocas.
Photo by Ryan Ellingson

5 (more) Misconceptions about Evolution

I recently came across this excellent post debunking 5 common misconceptions about evolution (hat tip to Joe Hansen at It’s Okay to be Smart). It’s a great infographic, and in light of the recent debate about whether creationism is compatible with science, between young-Earth creationist Ken Ham and Bill Nye the Science Guy, I thought I’d take this opportunity to add five more.

Full disclosure, the five misconceptions I’m writing about here are indeed common, but the inspiration was not all mine. This list is actually presented in The Tangled Bank, by the prolific science writer Carl Zimmer. It’s a great book for a non-biology major evolution course (I happen to be using it for just that right now), or for anyone outside the field who would like to learn more about evolution. No paid promo here, it’s just my opinion.

One thing that clearly unites every argument denying evolution: they are nearly always some form of a straw man argument. With ever-growing mountains of overwhelming evidence to support the theories behind evolution, this is really the only type of argument available to creationists. They must make false claims about evolution that they then easily take down to give their arguments the appearance of credibility.

strawman-macleod

Any time a creationist makes an argument to discredit evolution that seems on its surface to be scientifically compelling, this is probably how they did it. (Cartoon by history professor Dr. James MacLeod)

So in addition to the 5 misconceptions listed by Molecular Life Sciences, I figured it would be useful to add 5 more to the list (with no claims about whether these fit at the top, bottom, or middle of such a list). It turns out that most denials are so weak, they can be refuted in short order. So let’s get started…

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