An article in the Guardian says that at a conference next week, David Attenborough will voice his support for the aquatic ape hypothesis. I grew up watching Attenborough documentaries. I am a huge fan and would credit him with helping to ferment my interest in biology. But, I am no fan of the aquatic ape hypothesis because it is adaptationist and fails to provide parsimonious explanation for human evolution.
The aquatic ape hypothesis tries to force large number of human traits together under one umbrella explanation, that our ancestors had a close association with water. But no time period in the history of our evolution is specified and the fossil record shows that the traits claimed to have evolved in association with water appeared at widely different times. Without good fossil evidence demonstrating a strong association with water the hypothesis is dead... in the water.
The hypothesis is driving the evidence presented, not the other way around as it should be in science. A mish-mash of highly derived and rudimentary adaptations to water are used as evidence. Few of these are consistently associated with aquatic animals, such as hairlessness, which is present in several terrestrial mammals and absent in the majority of aquatic mammals. There are also a number of features that we humans have that are inconsistent with aquatic ancestry, such as internal testicles.
I am at a loss to explain how the aquatic ape hypothesis keeps getting coverage in popular press given how weak it is as an explanation. I get that human evolution is interesting, but it is such a bad explanation on the basis of both evidence and the methodology of its proponents. It's the phlogiston of explanations for the evolution of human traits. Fortunately, the recent coverage has spawned some well deserved ridicule, which has had a strong response on Twitter.
There is nothing unusual about primates exploiting aquatic resources for food. Wading bipedally in shallow water and even diving underwater for aquatic plants or shellfish in freshwater and in marine environments has been observed in many monkeys and apes and, of course, is common in primitive human populations. So its certainly not a question as to whether such bipedal aquatic feeding behavior in humans and other primates is possible.
ReplyDeleteThe question is, is there any evidence that a primate species actually became-- specialized-- in such aquatic feeding behavior for an extensive period of evolutionary time and whether its possible humans could be descended from such primates.
The aquatic ape hypothesis was first conceived by Oxford marine biologist, Sir Alister Hardy, back in the 1920s. But he didn’t reveal his hypothesis to the public until 1960 during a lecture and then in an article in the journal, New Scientist.
Elaine Morgan first encountered the hypothesis after she read a synopsis of it in the Desmond Morris book, the Naked Ape. Then she wrote about it in her own book, the Descent of Woman in the early 1970s.
Basically, Hardy’s argument was that humans became bipeds and developed a thick subcutaneous fat because they needed to wade into shallow water in order to get access to shellfish.
I should note that aquatic wading is also one of the leading hypotheses for the origin of bipedalism in archosaurs (dinosaurs, birds, and crocodilians)
I think its pretty obvious that Oreopithecus evolved its bipedalism as a wading adaptation for exploiting aquatic plants during its 2 million years of isolation on the ancient Mediterranean island of Tuscany-Sardinia.
The lobulated medulla of the human kidney strongly suggest that humans were once specialized in consuming foods with an extremely high salt content. Since the African continent tends to be deficient in food resources with high levels of salt, human ancestors obviously evolved such kidneys along a marine coastline.
The fact that humans evolved particular characteristics for the same reasons that most other animals evolve those same characteristics really shouldn't be all that surprising, IMO.
Marcel F. Williams
A quick look at the literature shows that bipedalism in archosaurs evolved several times and all of the hypotheses I read about do not link its evolution to water. If you have a reference that does, I would be happy to read it and let you know what I think.
DeleteA quick look at the literature on Oreopithecus shows that it is generally accepted that it probably evolved bipedalism to increase the efficiency of locomotion. Again, if you have an article that contradicts this, I would be happy to read it and let you know what I think.
Your discussion of the kidney serves to highlight a point I made in the post. If humans are descended from ancestors that had such a strong association with salt water that they evolved a specialised kidney, why do many other traits show a lack of specialisation to aquatic environments? For instance, why are we not testicond given that every other aquatic mammal is?
It is not obvious that our kidneys, or any of our traits, evolved because of an association with water. The evidence for the aquatic ape hypothesis is pathetic and sometimes even pseudo-scientific. You need to do better than provide circumstantial evidence. You need to provide evidence that is not only consistent with the aquatic ape hypothesis, but also rules out the currently better supported hypotheses.
References to the aquatic origin of bipedalism in archosaurs can be found in:
ReplyDeleteRomer, A.S. (1966) Paleontology, Chicago: The University of Chicago Press.
Neill, W.T. (1971) The Last of the Ruling Reptiles, New York: Columbia University Press.
Desmond, A.J. (1976) The Hot-Blooded Dinosaurs, New York: The Dial Press/James Wade.
Halstead, L.B. and Halstead, J. (1981) Dinosaurs, Poole, Dorset: Blandford Press.
Williams MF. The adaptive significance of endothermy and salt excretion amongst the earliest archosaurs. Speculat Sci Technol 1997; 20:237–47.
Oreopithecus swamp habitat:
Harrison & Rook, Function, Phylogeny, and Fossils: Miocence Hominoid Evolution and Adaptations. 1997
“The remains of Oreopithecus bambolii are extremely abundant in VI, and this species represents one of the commonest mammals at the site…..Evidence for a primarily aquatic setting and a humid forested environment is provided by the extensive lignite accumulations, the common occurrence of skeletal remains in anatomical connection, the abundance of fossil crocodiles, chelonians, and freshwater mollusks, and the occurrence of otters…..The area was evidently poorly drained, and the forested areas were interspersed with numerous freshwater pools and shallow lakes. pg 335
“Interestingly, there is also a corresponding decline in the abundance of Oreopithecus in V2, which might simply imply a relatively narrow ecological preference by this taxon for swampy, forested habitats.” pg. 336
“Another possibility is that Oreopithecus was exploiting aquatic or wetland plants, such as water lilies, reeds, sedges, cattail, pond weeds, horsetails, and stoneworts, all of which are abundantly represented in the pollen spectrum from Baccinello.” pg. 341,
And yes it is obvious that human kidneys are adapted to excreting high levels of salt. Humans are the only Catarrhine primate with lobulated medullas in their kidneys, a feature that is universal in marine mammals but also found in terrestrial desert mammals that drink brine with a salt content higher than seawater. I've published two articles on this subject.
"And yes it is obvious that human kidneys are adapted to excreting high levels of salt."
DeleteI don't dispute that human kidneys are capable of excreting high levels of salt. What is not at all obvious is that this has evolved as an adaptation to life in marine environments. As you point out, there are other reasons that such adaptations might arise.
I'll try and get a hold of the references you cite, but I don't like my chances given their age. Not exactly the up-to-date, cutting-edge research I was hoping for. Also, my institution doesn't subscribe to "Speculations in Science and Technology". But, it too, is old and it's only been cited twice, by its author (you). That does not give me confidence that it presents a compelling case. You could send it to me [the name of this blog as one word] at gmail dot com.
I've done a little bit of reading on kidneys and it's pretty clear that your claims are wrong.
DeleteFirstly, lobulate kidneys are not universal in marine mammals. The sirenia do not have them.
Secondly, lobulate kidneys are present in many terrestrial mammals. Interestingly, they are relatively common in ruminants, which are more closely related to whales and dolphins than other mammals. And lobulate kidneys are present in bears, which are the closest relatives of the seals. So, kidney morphology in aquatic mammals appears to be better explained as shared common ancestry than adaptation to marine environments.
Finally, kidney morphology is not correlated with excretion efficiency in marine mammals. Hormonal mechanisms are more likely to regulate the salt concentration in urine.
We did not descend from aquatic apes, of course, although our ancestors were anatomically & physiologically not adapted to running over open plains as some anthropologists still believe. Instead, Pleistocene Homo populations simply followed the coasts & rivers in Africa & Eurasia (800 ka they even reached Flores >18 km overseas), google, eg, “econiche Homo”.
ReplyDelete–eBook “Was Man more aquatic in the past?” introd.Phillip Tobias
http://www.benthamscience.com/ebooks/9781608052448/index.htm
–guest post at Greg Laden’s blog
http://scienceblogs.com/gregladen/2013/01/30/common-misconceptions-and-unproven-assumptions-about-the-aquatic-ape-theory
Hi Marcel,
ReplyDeleteOur conference earlier this month in London with David Attenborough & Donald Johanson was a great succes. It's clear that archaic Homo during the Pleistocene followed the coasts & rivers when they trekked to England (Boxgrove, Pakefield), the Cape & Indonesia (Java & Flores).
Human newborns have renculated kidneys (fully subdivided kidneys, often seen in marine mammals), but older humans have kidneys more resembling those of pigs & beavers (plenty of freshwater). This suggests that after a littoral phase (coastal diaspora of erectus-like people), early sapiens ventured inland along rivers, beaver ponds, reedbeds, lakes etc., presumably first during warmer periods (Summer? intergacials?), later permanently.
Sure, different representatives of Homo may well have stuck to the coasts when they migrated out of Africa. But I still have not seen any evidence that convincingly demonstrates that any adaptations in Homo, or earlier Hominids, are a result of life in aquatic environments. It's speculation based on circumstantial similarities between some features of humans and some aquatic mammals. You have to pick and choose which mammals to compare with which features. You also have a mosaic of highly specialised features and rudimentary features. It's an inconsistent, mish-mash that's rich in assumptions and deficient in explanatory power.
DeleteIt seems you are also resurrecting Haeckel's defunct "ontogeny recapitulates phylogeny" hypothesis in your second paragraph.