Living fossils can evolve quickly

The evolution of living fossils is not unusual. The odd thing about them is that they have ancient origins and are, today, not very diverse groups. Sturgeon are ray-finned fishes in the family Acipenseridae, which are known from almost 200 million year old fossils. Fossils that are recognisably similar to modern sturgeons appear about 100 million years ago. There are 23 (but maybe a few more) modern species in the Acipenseridae, which is probably fairly similar to their historical diversity.

A sturgeon in the genus Acipenser, possibly A. transmontanus (image Wikipedia).

Other families of fish are much more diverse than the Acipenseridae. Some of the most beautiful marine fish are in the families Gobiidae (gobies) and Labridae (wrasses), with over 2,000 and 600 species respectively. And they aren't even the largest families of fish. That honour probably goes to the Cyprinidae (carps and minnows) with over 2,400 species. This pattern raises an interesting question; why are some groups highly diverse while others are not?

The old glory goby, Amblygobius rainfordi (top), the Yellowtail coris wrasse, Coris gaimard, being cleaned by a cleaner wrasse, Labroides phthirophagus (middle), and the cleaning goby, Elacatinus evelynae (bottom). Notice the convergent colour evolution in the cleaner fishes (all images Wikipedia).

One hypothesis is that some groups are more 'evolvable'. These groups are able to change their morphology relatively quickly and are consequently able to form more new species. For this hypothesis to be true, we should see a correlation between the amount of phenotypic change within a group and the amount of speciation. Studies that have looked at this relationship within lineages have failed to detect it.

A new paper by Rabosky and others argues that a fairer test of the hypothesis is to look at the relationship among lineages. They constructed an enormous evolutionary tree containing almost 8,000 species from across the entire spectrum of diversity of ray-finned fishes. They used body size evolution as a proxy for phenotypic change and found overwhelming support for the hypothesis; families with greater diversity in body size typically contained a greater number of species.

One group to buck this trend were the sturgeons. The rate of change in body size in sturgeons is amongst the fastest for all fishes. The baluga, Huso huso, is one of the largest ray-finned fishes, growing to over five and a half meters and 1,000 kilograms, while the dwarf sturgeon, Pseudoscaphirhynchus hermanni, grows to just 27 cm and 50 grams. This considerable difference in body size has arisen almost 5.4 times more rapidly than expected and only five of the 172 families in the analysis diverged faster.

These results are in complete contrast to the popular conception of living fossils as slow changing evolutionary oddballs. Indeed, in this analysis sturgeon are outliers because of the speed of their evolution. Clearly some traits in sturgeon are highly conserved over large periods of time, but we should not assume that this means that all of their traits are highly conserved. Living fossils are evolving and can diverge rapidly.

I've also written about living fossils here and here.

Reference

Rabosky, D., Santini, F., Eastman, J., Smith, S., Sidlauskas, B., Chang, J., & Alfaro, M. (2013). Rates of speciation and morphological evolution are correlated across the largest vertebrate radiation Nature Communications, 4 DOI: 10.1038/ncomms2958

Worm sperm

You may have never thought about what feature distinguishes males from females. After all, in mammals the differences are often clear to us. In other groups too, the differences between male and female traits are often conspicuous. But, there are many species where male and female reproductive organs are both present in the same individual. Even in these species we can tell male parts from female parts.

To distinguish male from female we look at the relative size of the sex cells or gametes. Males produce the smaller gametes (e.g. sperm) and females produce the larger gametes (e.g. eggs). This difference in the size of the gametes is known as anisogamy, which essentially means without ("an") the same ("iso") gametes ("gamy"). 

The converse of anisogamy is isogamy. Species that are isogamous are very rare now, but this is thought to be the ancestral condition. As in anisogamous species where fertilisation only occurs when egg and sperm meet, fertilisation cannot occur in isogamous species unless the gametes of two different mating types meet. In isogamous species mating types are are referred to by various names, such as "+" and "-", in place of male and female.

The origins of anisogamy are unclear, but we have a pretty good explanation for why it evolved. Each gamete an individual produces costs energy and it must be stocked with additional reserves so that the zygote can complete development and start acquiring it's own energy. In isogamy, each member of a pair contributes half the energy to produce a viable offspring. In anisogamy, the cost is overwhelmingly paid by one of the mating types.

Investing almost nothing in individual gametes comes with a huge advantage, vastly more gametes can be produced increasing the number of offspring you can potentially produce. The more gametes an individual has the more fertilisations and individual can potentially achieve. Once one mating type gets far enough down the path of small gametes, its pair can't follow because that is likely to result in a zygote that doesn't have enough resources to survive.

It is relatively clear that fertilisation success has driven the evolution of males that produce more, small sperm. However, there are other aspects of sperm size and shape that appear to contribute to fertilisation success and these are surprisingly variable among and within species. Clear demonstrations that differences in sperm characteristics affect fertilisation success are rare, which makes a new paper in Evolution particularly interesting. 

Darren Johnson of the National Centre for Ecological Analysis and Synthesis, with Keyne Monro and Dustin Marshall of UQ (now both at Monash) looked at sperm traits in the broadcast spawning tubeworm, Galeolaria gemineoa. These worms can occur individually or in huge aggregations, leading to substantial variation in the concentrations of sperm and eggs in the wild. Because they don't leave their tubes, their options for increasing fertilisation success are limited relative to mobile species.

A colony of Galeolaria caespitosa, which are nearly identical to G. gemineoa (photo D. Semmens).

Groups of eggs from multiple females were exposed to the sperm of a single male at six different concentrations and two different ages. Fertilisation success was measured at the proportion of eggs that were undergoing normal development within each treatment. This is not a direct measure of fertilisation success because some embryos may have died very early due to genetic incompatibilities rather than the absence of fertilisation. However, it is a reasonable and practical proxy.

At high sperm concentrations, males that produced sperm with longer average tail length and smaller average head size achieved greater fertilisation success. In contrast, males that produced sperm with longer than average heads were favored at low sperm concentrations and older age. The results suggest that variation in sperm size and shape within a species may be preserved because different fertilisation environments favor contrasting sperm characteristics. 

The logistics of genetically assigning paternity prevented the authors from varying sperm competition environments. Had the sperm of multiple males been in competition to fertilise the eggs, different traits or trait combinations could have been favoured. While it is probably more realistic to pit the sperm of several males against each other, single male experiments still provide useful insights into selection on sperm traits.

An abbreviated version of this post also appears in the Research Highlights on the Australasian Evolution Society website.

References:

Johnson, D., Monro, K., & Marshall, D. (2013). The maintenance of sperm variability: Context-dependent selection on sperm morphology in a broadcast spawning invertebrate Evolution, 67 (5), 1383-1395 DOI: 10.1111/evo.12022

Living fossils are evolving

Charles Darwin coined the term living fossil in On the Origin of Species. He didn’t use it the same way that it has come to be used. He suggested that living fossils are modern species that can be used to link to groups in the same way that fossils can. One of the examples he gave was the platypus, which lactates and lays eggs, which is evidence that mammals and reptiles share a common ancestor. I don't think he meant it to mean an unchanged relict, as some people interpret his words.

Today, a living fossil is a species that retains many features of their fossil ancestor so that it is recognisably closely related. There are some stunning examples of this, such as orb-weaving spiders. In 2011 a 165 million year old spider fossil was described by Seldon et al., which shared so many features with modern Nephila spiders that it was placed within the same genus. Interestingly, I have never heard of web building spiders being referred to as living fossils despite there being amazing conservation of traits in many groups.

The orb-weaving spiders Nephila clavipes (left) and N. jurassica (right) are separated by 165 million years, but placed within the same genus (image of N. clavipes from Wikipedia and N. jurassica from Seldon et al. 2011).

Unfortunately, living fossil has become synonymous with a species, or group of species, displaying no evolutionary change or very slow change. This is completely wrong. Although the conservation of morphology in Nephila is remarkable, there are more than 150 known species in the genus. Clearly there has been evolutionary diversification within the group. Indeed, whenever living fossils are examined in more than superficial detail it becomes difficult to see them as organisms that evolution forgot.

Horseshoe crabs are one of the most iconic living fossils. There are four living species in three genera. They are placed within the subphylum chelicerata, which makes them more closely related to spiders and scorpions than they are to true crabs, which are placed within the subphylum crustacea. Although there are fewer species of horseshoe crabs than Nephila, that fact that there are four species that are all different from fossil species is a strong indication that evolution hasn't stopped for them.

The Atlantic horseshoe crab, Limulus polyphemus, mating (photo Wikipedia)

The general shape of modern horseshoe crabs is strikingly similar to the fossils that date from about 450 million years ago. Close examination, though, shows that parts of their shape, their legs in particular, have changed over time. Briggs et al. 2012 looked at a fossil horseshoe crab from 425 million years ago, which is relatively early in their evolution. They found that modern horseshoe crabs are missing an entire set of limbs that were present in their ancestors.

All modern chelicerates, including living horseshoe crabs, have unbranched limbs; each limb is a single series of segments. Most crustaceans have limbs that branch at the base into two series of segments. Branched limbs, like those in crustaceans, are the ancestral condition and unbranched limbs are thought to have evolved several times among the arthropods. Indeed, Briggs et al. found that the fossil horseshoe crab had branched limbs, which have been lost in their descendents. 

Like horseshoe crabs, tadpole shrimp have a broad semi-circular carapace protecting their heads and are considered living fossils. There are 11 recognised species in two genera, Lepidurus and Triops. The two genera probably diverged about 180 million years ago, but there are fossil tadpole shrimp dating from about 250 million years ago. That's not as long as the really iconic living fossils, like horseshoe crabs and the coelacanths, but it is still an impressive amount of time to retain enough features to be easily recognised as related.

The tadpole shrimp, Lepidurus apus (photo Wikipedia)

The problem with relying on features that preserve in the fossil record is that it underestimates the actual amount of evolutionary change because generally only hard parts are preserved. A recent study of tadpole shrimp highlights this point. Mathers et al. 2013 used genetic analyses to construct the evolutionary relationships among the 11 species of tadpole shrimp. They found that there are actually 38 species and that these species arose relatively recently. This shows that rather than evolutionary stasis, there is likely to be high species turnover in the group.

There are many reasons why some features may be conserved over long periods of time. None of these have to do with natural selection taking a break. In fact, if natural selection did cease we should expect to see features wander under random genetic drift, as has been hypothesised for eyes in cave dwelling animals. Conserved features are much more likely to be the result of developmental constraints or stabilising selection.

References:

Briggs, D., Siveter, D., Siveter, D., Sutton, M., Garwood, R., & Legg, D. (2012). Silurian horseshoe crab illuminates the evolution of arthropod limbs Proceedings of the National Academy of Sciences, 109 (39), 15702-15705 DOI: 10.1073/pnas.1205875109 

Mathers, T., Hammond, R., Jenner, R., Hänfling, B., & Gómez, A. (2013). Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’ PeerJ, 1 DOI: 10.7717/peerj.62

Selden, P., Shih, C., & Ren, D. (2011). A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China Biology Letters, 7 (5), 775-778 DOI: 10.1098/rsbl.2011.0228

In the cave of the blind, the no-eyed crab is king

Cave dwelling creatures are often blind. The prevailing view is that, in such species, mutations in the visual system have little or no effect on fitness and vision is lost as these mutations gradually accumulate. There are several other types of characters that we can be reasonably confident are adaptations to life in caves, such as elaboration of structures for touch or smell. However, it is often hard identify which population cave adapted species are descended from and, therefore, how long ago they invaded caves. Without this information it has been hard to test ideas about the evolution of traits associated with life in the dark.

A cave form of the fish, Astyanax mexicanus, which is eyeless and unpigmented, traits typical in caves. It is a commonly used model species in studies of adaptation to cave environments (photo Wikimedia Commons).

Sebastian Klaus and colleagues from the National University of Singapore and Goethe University examined five species of freshwater crab in the genus Sundathelphusa, which occur on Bohol Island in the Philippines. Four species are only found in caves and the other has established several populations in caves. The repeated invasion of caves by the crabs has led to varying degrees of adaptation to life in the dark within the group. 

Freshwater crabs in the genus Sundathelphusa from Bohol Island. Thy are arranged from least cave adapted (top) to most cave adapted (bottom). From top to bottom the species are Sundathelphusa boex, S. vedeniki, S. urichi, S. sottoae and S. cavernicola (from Klaus et al. 2013).

The team used genetic data to estimate the time at which each species and population last shared a common ancestor. They then compared several features of cave-adapted crabs with their closest terrestrial relatives. Reductions in the visual system were just as pronounced as changes in cave-adapted features, indicating that evolution occurs at similar rates. The authors argue that this is a clear sign that eye loss is under directional selection because changes should appear more slowly if they are a result of selectively neutral mutations. 

They don’t speculate at all about what might favour eye-loss in the Bohol crabs, but hint in the introduction that it could be due to trade-offs between vision and other sensory systems. Trade-offs occur where increasing one aspect of fitness necessarily requires the reduction of fitness in another. If eyes are energetically costly to build and maintain then retaining functional eyes might prevent greater investment in other senses. Trade-offs are ubiquitous in biology and have been implicated in the loss of eyes in other cave dwelling species.

While I was doing research on this study I came across several creationist websites that argue cave adapted creatures are strong evidence that evolution is false because a trait is lost. According to them this shows evolution progressing in the wrong direction to what is predicted. They argue that evolution should progress towards more information and greater complexity. This is incorrect and shows, yet again, that creationists typically have a poor understanding of evolutionary theory.

The 'logic' of this argument is similar to the idea of a "Great Chain of Being", which pervaded early thinking about biology. This type of thinking is where we get several antiquated, but persistent terms, such as "missing link" and "highly evolved". It continues to dog evolution in the way that evolutionary information is often presented, such as the placement of organisms more closely related to us at the right or top of phylogenetic trees and at the end of textbooks.

The phylogeny of primates with humans at the top and less related groups at the bottom (from Wikipedia).

Linear descent was never part of Darwin's theory, nor was an increase in information ever a necessary assumption on which evolutionary theory rests. When you look at an evolutionary tree (like the primate tree above), all of the living species at the branch tips have an equally long evolutionary history. They are not descended from each other, they are descended from a common ancestor. You could say that they are equally evolved.

The first evolutionary tree drawn by Darwin over 20 years before the publication of On the Origin of Species.

Evolution doesn't prevent information from increasing, but contrary to the creationist claims it does predict that there will be strong limits on it. Both single trait and multi-trait trade-offs are thought to prevent organisms from becoming perfectly adapted. Single trait trade-offs occur where elaboration of a structure increases fitness in one environment, but reduces it in others. Multi-trait trade-offs occur where two or more structures are dependent on a shared finite resource.

Blind crabs are not evolving in the wrong direction. There is no wrong direction, they're just evolving under the constraint of trade-offs. Eye reduction and loss of pigmentation are not the only evolutionary changes that are occurring either. Other traits are becoming more elaborated, such as the length of their legs and the hairs on their claws, suggesting a multi-trait trade-off. This result is not only consistent with evolutionary theory, but expected.

An abbreviated version of this post is published on the Australasian Evolution Society website in the Research Highlights section.

Reference

Klaus, S., Mendoza, J., Liew, J., Plath, M., Meier, R., & Yeo, D. (2013). Rapid evolution of troglomorphic characters suggests selection rather than neutral mutation as a driver of eye reduction in cave crabs Biology Letters, 9 (2), 20121098-20121098 DOI: 10.1098/rsbl.2012.1098

Research Highlights from the Australasian Evolution Society

I have been asked by the Australasian Evolution Society to provide some 'Research Highlights' for their newly launched website. The Research Highlights promote interesting recent research by evolutionary biologists in Australasia. To get more diversity in the types of research covered there will be two or three others writing too. My stories will go up every few weeks and I will endeavor to publish them here as well, probably with some additional comments. My first piece went up a few weeks ago and I've submitted my second, which should go up shortly. I'll post here as soon as it is.

My favourite science books

I've been talking to a few people recently about good science books. My favorite books that deal with similar topics to this blog are in descending order:

Mapping the deep - Robert Kunzig

The World without us - Alan Weisman

The wavewatcher's companion - Gavin Pretor-Pinney

Trilobite - Richard Fortey

At the water's edge - Carl Zimmer

Books outside the topic area of this blog that I found to be excellent are:

The demon-haunted world - Carl Sagan
The elegant universe - Brian Breene
Chaos - James Gleick

Neil Shubin's "Your inner fish" is sitting on my bookshelf just waiting to be read. I hear it is very good and will probably make it onto my list. Several books by Dawkins and Gould are also among my favorites, but I liked them less than the ones above. The ancestor's tale (Dawkins) and Wonderful life (Gould) are probably the best I've read of their books.

Physics versus biology

When I was in high school, my physics teacher drew a massive rectangle that took up almost the entire black board and proclaimed, "this is physics". He then drew a little rectangle inside the first one and proclaimed, "this is biology". The he made a tiny little smudge of chalk on one of the sides of the 'biology' rectangle and said, "this is chemistry, so you all should study physics because all the other major division of science are just sub-disciplines of physics".

Amusing as his performance was, there are many aspects of biology that cannot be directly or indirectly inferred from our understanding of physics. Notably, we would have never formulated the theory of evolution, which underpins our modern understanding of biology, if we had to rely on progress in physics alone. Clearly though, physics is important in shaping the evolution of particular traits. I've written many times about physics in biology, such as swimming in sharks (here), flight in albatross (here), the hammer strike of mantis shrimp (here) and the visual capability of giant and colossal squid (here and revisited here).

Almost all organisms that detect and use light do so in the same part of the spectrum, which is pretty much the same part of the spectrum we see in. Although many use slightly shorter wavelengths in the ultraviolet or slighter longer wavelengths in the infrared, no organisms that we know of use the huge parts of the spectrum in the radio, x-ray and gamma ray wavelengths. I've wondered why this is before, but Mathew Cobb wondered it out loud and got some interesting answers.

A stepping stone of rotting wood

Many of the animals living at hydrothermal vents and cold seeps carry chemosynthetic bacterial symbionts in their body, which convert methane or hydrogen sulfide into food. Some have lost the ability to feed on anything other than what the bacteria living inside their tissues provide them. Almost all cannot survive without a sufficient supply of methane or hydrogen sulfide. One hypothesis is that decomposing organic matter that has sunk from the surface, like whale carcasses, seaweed, and wood could serve as a food source, providing stepping stones between vents or seeps.

A field of mussels at a cold seep (photo Wikipedia)

Animals more typically found at vents and seeps are known to colonise the remains of whales in the deep sea. Smith et al. (1989), were the first to report vent animals colonising whale skeletons. They also provided a conservative estimate of the distribution of whale carcasses on the ocean floor and suggested that they would provide a persistent and abundant habitat for cold seep and hydrothermal vent animals. Adding sunken wood and seaweed to the list only increases the amount of available habitat.

A whale skeleton in the deep sea with patrolling hagfish (photo Wikipedia)

A gap in our understanding, though, is how enough methane or hydrogen sulfide is produced to support populations of chemosynthetic animals by decaying wood. The deep sea is a cold place, which is not conducive to the rapid breakdown of organic material. A large amount of wood would be required to provide the surface area necessary to produce enough gas. One hypothesis though, is that the surface area of wood might be increased by larger organisms, such as wood-boring bivalves, breaking it up first.

Researchers tested this idea by depositing wood logs on the Eastern Mediterranean seafloor at 1700 meters down and returned a year later to examine the animals and bacteria the had colonised the wood. They also measured the chemicals in the water released by the bacteria breaking down the wood. Using underwater robots, they observed that wood-boring bivalves had indeed broken the wood into smaller pieces, which were further broken down by other organisms. 

The activity of the organisms digesting the wood reduced the amount of dissolved oxygen, resulting in anoxic conditions that allowed sulfate-reducing bacteria to move in and produce hydrogen sulfide. The hydrogen sulfide then attracted a species of mussel, which usually found at cold seeps where it gains energy from symbiotic chemosynthetic bacteria. The mussels seemed to preferentially colonise cavities under the bark of the wood, presumably because sulfide levels were higher there.

The chemosynthetic mussel Idas modiolaeformis was found in the sunken wood piles (photo from Bienhold et al. 2013)

So it appears that wood boring organisms are able to pave the way for chemosynthetic organisms to colonise sunken wood in the deep sea. Their burrows, feces and the chips of wood that they produce all increase the surface area of material available for hydrogen sulfide producing bacteria to digest. Moreover, their activity and the activity of other organisms produce the anoxic conditions required for sulfate reduction, which is necessary to support chemosynthetic life.

A hypothetical succession of animals on submerged wood in the deep sea over a year. Initially wood-boring bivalves move in, followed by predators and detritus feeders (e.g. polychaetes and sipunculids). The respiration of the colonisers creates anoxic niches that allow the chemosynthetic mussels to move in (diagram from Bienhold et al. 2013).

Smith, C., Kukert, H., Wheatcroft, R., Jumars, P., & Deming, J. (1989). Vent fauna on whale remains Nature, 341 (6237), 27-28 DOI: 10.1038/341027a0  

Bienhold, C., Pop Ristova, P., Wenzhöfer, F., Dittmar, T., & Boetius, A. (2013). How Deep-Sea Wood Falls Sustain Chemosynthetic Life PLoS ONE, 8 (1) DOI: 10.1371/journal.pone.0053590

Evolution, climate change and coral

Increased carbon dioxide in the atmosphere poses several problems for organisms living in the marine environment. Increases in temperature and ocean acidification are the two best known and most worrying. In order to predict how climate change and ocean acidification will affect marine species, we need to know how they respond to these conditions. The effect of climate change on corals has attracted a lot of attention because of their importance for biodiversity.

We can't just expose corals to predicted conditions because corals of the future won't be naive to these environments and are likely to have evolved. We know that evolution can be extremely rapid, often within decades. Ignoring the potential for evolution to influence the effects of climate change on marine organisms could lead to inaccurate projections of the effects of climate change on extinction risk. Yet many authors are ignoring the effects of evolution and acclimation in making their predictions. 

The three-spine stickleback, Gasterosteus aculeatus has been documented adapting to freshwater conditions from saltwater ancestors in just 13 generations (photo Wikipedia)

In their 2007 paper, Hoegh-Guldberg, et al. dismiss the importance of evolution because "reef-building corals have relatively long generation times and low genetic diversity, making for slow rates of adaptation". But, long generation times are not present in all coral species and the response of corals to climate change is going to depend partly on their algal symbionts, which have short generation times. 

Unfortunately, the rates of evolution in corals and their symbionts are extremely poorly known. In terrestrial systems though, genetic variation for traits related to thermal performance is common and evolutionary responses to changing climate are typical. For instance, changes in allele frequencies consistent with responses to global warming have been documented in a number of insects, such as fruit flies and mosquitoes (e.g. Bradshaw & Holzapfel 2001, Umina et al. 2005).

Acclimation, or phenotypic plasticity, will also affect the way that corals respond to climate change. Plastic responses to the environment can occur within generations and across them. For instance, Donelson et al. (2011) looked at the tropical damselfish, Acanthochromis polyacanthus, and found that their offspring could completely compensate for the negative effects of higher temperatures. But, this only occurred when both they and their parents where reared at the same temperature.

The tropical damselfish, Acanthochromis polycanthus (photo Wikipedia)

There are indications that some acclimation is occurring in corals too. Under stress, corals expel their algal symbionts, which gives them the appearance of having been bleached. Coral reefs that experience greater variability in sea surface temperature and those that have recently been subjected to bleaching are less susceptible to bleaching. This greater resilience suggests that some acclimation to climate change is possible within short time-frames. 

A bleached coral in the foreground with an unbleached coral of the same species behind (photo Wikipedia)

We need a better understanding of how evolution and acclimation may influence the response of corals to climate change so that our predictions are accurate. But, we already know which direction things are probably going to go. John Pandolfi's work has shown that under historical climate change, diversity on corals reefs has declined and populations have moved to higher latitudes (e.g. Pandolfi et al. 2011, Kiessling et al 2012). 

Climate change is currently more rapid than previous episodes and this will limit the amount of adaptation that can occur. Corals are also already under significant pressure from other anthropogenic sources of stress that have resulted in substantial declines and changes in population composition. These pressures, too, will decrease the ability of corals to cope with the effects of climate change. By removing these pressures, we will give corals the best chance possible to adapt to a warmer and more acidic ocean.

Hoegh-Guldberg, O., Mumby, P., Hooten, A., Steneck, R., Greenfield, P., Gomez, E., Harvell, C., Sale, P., Edwards, A., Caldeira, K., Knowlton, N., Eakin, C., Iglesias-Prieto, R., Muthiga, N., Bradbury, R., Dubi, A., & Hatziolos, M. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidification Science, 318 (5857), 1737-1742 DOI: 10.1126/science.1152509  

Bradshaw, W., & Holzapfel, C. (2001). Genetic shift in photoperiodic response correlated with global warming Proceedings of the National Academy of Sciences, 98 (25), 14509-14511 DOI: 10.1073/pnas.241391498

Umina, P., Weeks, A. R., Kearney, M. R., McKechnie, S. W., & Hoffmann, A. A. (2005). A Rapid Shift in a Classic Clinal Pattern in Drosophila Reflecting Climate Change Science, 308 (5722), 691-693 DOI: 10.1126/science.1109523

Donelson, J., Munday, P., McCormick, M., & Nilsson, G. (2011). Acclimation to predicted ocean warming through developmental plasticity in a tropical reef fish Global Change Biology, 17 (4), 1712-1719 DOI: 10.1111/j.1365-2486.2010.02339.x

Pandolfi, J., Connolly, S., Marshall, D., & Cohen, A. (2011). Projecting Coral Reef Futures Under Global Warming and Ocean Acidification Science, 333 (6041), 418-422 DOI: 10.1126/science.1204794 

Kiessling, W., Simpson, C., Beck, B., Mewis, H., & Pandolfi, J. (2012). Equatorial decline of reef corals during the last Pleistocene interglacial Proceedings of the National Academy of Sciences, 109 (52), 21378-21383 DOI: 10.1073/pnas.1214037110

The aquatic ape hypothesis is rubbish

I mentioned the aquatic ape hypothesis in a recent post on wrinkly fingers improving grip when handling submerged objects. As if on cue, I then received this email:

Dear colleague,

Humans are very different from other primates, and many evolutionary hypotheses have been proposed to explain this remarkable fact. Is there scientific consensus on which of those hypotheses are most substantiated? If not, do opinions differ among researchers with different backgrounds?

Please help to establish the state of the art by answering a survey on this topic. You receive this invitation because you have recently published in a scientific journal covering paleoanthropology, paleontology or (human) biology, which indicates that you have expertise in a relevant field. 

A link to the survey is provided in the end of this message. 

Your responses will be anonymous. If you wish to be informed about the outcome of the survey, you can add your e-mail address to the mailing list after having completed the main survey. The present mailing list will only be used to send the invitations. My apologies if you receive this message more than once. 

Thank you for participating! 

Yours sincerely,
[Name and contact details redacted]

The first couple of pages were reasonable enough, containing questions on a variety of theories for how human traits evolved. Which is as you would expect from a survey that is trying to "establish the state of the art" on human evolution. But, the survey then degenerated into asking questions solely about the aquatic ape hypothesis. A quick bit of googling later and I find that the sender is a proponent of the hypothesis.

I am baffled by the persistence of the aquatic ape hypothesis. It has received almost no attention in the scientific literature because on more then superficial examination it fails to provide a parsimonious explanation of human evolution. In fact, to accept the aquatic ape hypothesis you would not only have to find its scant evidence convincing, but assiduously ignore the contradictory evidence.

Take the claim that bipedalism evolved in our ancestors as an adaptation to an aquatic conditions, from a situation much like how gorillas and chimpanzees walk when wading. It's a fine and very plausible claim, but it doesn't fit with what we know from the fossil record. All of the putative human ancestors, such as Australopithecus and Orrorin, retain features that show they were still capable tree climbers when they first started walking upright. For the aquatic ape hypothesis to be true, they would have had to come to the ground first then moved to water before becoming bipedal. There is nothing to support this in the fossil record.

Another problem that the human fossil record poses for aquatic ape proponents is that several of the traits claimed to be adaptations to aquatic conditions appeared separately. For instance, it's claimed that only a seafood diet rich in omega-3 fatty acids could support the expansion of brain size in the human lineage. Brain expansion, however, came after bipedal locomotion, indicating that the association with water had to have lasted several million years for both of these claims to be true.

Hairlessness is one of the traits that is central to the aquatic ape hypothesis. Again it is a fine and plausible claim to make, but hairlessness is poorly associated with an aquatic lifestyle. There are some notably hairless aquatic mammals, such as whales, dolphins and walruses, but there are many more aquatic animals that retain hair. Moreover, there are some hairless mammals that are clearly not aquatic, such as naked mole rats, rhinoceros[1] and elephants. Hairlessness is clearly neither necessary for an aquatic life nor is it uniquely associated with aquatic mammals.

A feature that is far more commonly associated with aquatic mammals is that they are testicond, that is they have internal testicles (at least the males do!). In fact, I know of no aquatic mammal that is not testicond. There are some mammals that are not aquatic and are also testicond, like our friends the rhinoceros and elephant[2], but the far more common condition for terrestrial mammals is to have external testicles. So, the external testicles of human males suggests that humans are typical terrestrial mammals and not aquatic apes.

There are a whole host of other features that aquatic ape proponents cite as evidence consistent with aquatic ancestry and it would take an entire website to debunk them all (like this one). Individually, none of these traits are very compelling evidence for their hypothesis, and taken together they're a complete mish-mash. While some are highly derived features, like hairlessness, others have barely changed from the ancestral condition, like webbing between the fingers.

More parsimonious explanations for the evolution of human traits involve multiple causes, not a single cause as proposed by the aquatic ape hypothesis. The aquatic ape hypothesis is neither simple[3], nor does it fit well with the available evidence. Proponents of the aquatic ape hypothesis need to bend the evidence to fit their hypothesis and resort to special pleading to explain away the inconsistencies in their arguments. It's a textbook example of adaptationist just-so storytelling.


[1] An interesting aside is that the species of rhinoceros that is most closely associated with water is often called the hairy rhinoceros.

[2] Another interesting aside is that both the rhinoceros and elephant are thought to have aquatic ancestors. The support for this is much stronger for the elephants, which are part of a clade (the Tethytheria) who's other members are all aquatic (the extant Sirenia and the extinct Desmostylia). This may explain why they and rhinoceroses are testicond. It does not explain their hairlessness, as we know that as recently as 10,000 years ago there were a number of very hairy rhinos and elephants, indicating that the common ancestors of modern rhinos and elephants were also hairy.

[3] The transition from arboreal to terrestrial to aquatic back to terrestrial is not a simple evolutionary scenario. A simpler assumption would be that human ancestors were all terrestrial after descending from the trees.