In search of "Eohomo"

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During his 1876 tour of the United States, the famed anatomist and popularizer of science Thomas Henry Huxley stopped to see the American paleontologist O.C. Marsh at Yale. Marsh provided his esteemed guest with access to his ever-growing stores in the Peabody Museum, showing Huxley toothed Cretaceous birds and an array of fossil horses that convinced Huxley that the horse was a creature that had evolved in the New World, not the Old. Indeed, Marsh had collected an impressive array of fossil horses, from tiny forms with many toes to the familiar one-toed Equus. Given the transitions that were apparent, Huxley thought that an even older horse with five toes might eventually be found, and he created a whimsical sketch of such an Eohippus (or "Dawn Horse") being ridden by an Eohomo (or "Dawn Man"). (A version of which was recently cast in bronze by Pangaea Designs.)

Huxley did more than just create cartoons, though. He revised his lectures to include Marsh's material, and proposed that eventually there would be found an ancestral horse with four toes on each foot (and a rudiment of a fifth). Shortly after Huxley suggested this hypothesis to his audiences, Marsh found that he had been in possession of Eohippus for some time (it was in his museum, encased in sediment), and when Huxley's American Addresses were printed a note was included to state that the discovery of Eohippus provided as good a confirmation of Huxley's hypothesis as could be hoped for.

The hypothetical rider of Eohippus, however, proved to be much more elusive. It was only in 1859 that the existence of "men among the mammoths" became a tenable idea, spurred on by studies of Pleistocene cave deposits in Europe. Even then most of the human fossils known from the 19th century through the early 20th were from anatomically modern humans and Neanderthals (which were often taken to be pathological or otherwise aberrant Homo sapiens), providing few clues of the actual ancestry of humans. The outlier was the Pithecanthropus erectus (now Homo erectus) discovered by Eugene Dubois in Java, but opinions were divided and Dubois kept the fossil hidden away for a time as he worked out how it could be considered to be a human ancestor based upon brain and body size. If life had evolved certainly humans could not be exempt, but researchers had few details to work with.

Given the general dearth of fossil evidence, scientists often compared our species to living and fossil apes in order to determine our place in the tree of life. We may take it for granted today that our species is most closely related to chimpanzees, having shared a common ancestor with them between about 7 and 5 million years ago, but this has only become resolved very recently through the comparison of fossil and molecular evidence. Our family resemblance to apes has long been recognized, but the details of that relationship were vague for a long time. This allowed different researchers to draw of differing bodies of evidence and interpret it in different ways.

One of the most intriguing trends in hypotheses of human evolution around the turn of the 20th century was the proposal of some scientists that living and fossil apes provide us with almost no clue about our own ancestry. Chimpanzees, gorillas, and gibbons might represent models of our very distant ancestors, but to some scientists we had our own distinguished pedigree. In his very popular survey Men of the Old Stone Age, for instance, Henry Fairfield Osborn proposed that we shared some unknown common ancestor with living apes during the Pliocene, the gibbon most closely preserving our non-ape ancestor. Once our line started it was a straight march from "Pithecanthropus" through Homo sapiens. This made living apes of little use in determining our own evolutionary history, and Osborn stated;

Among these fossil anthropoids [apes], as well as among the four living forms, we discover no evidence of direct relationship to man but very strong evidence of descent from the same ancestral stock.

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Men of the Old Stone Age Their Environment, Life, and Art By Henry Fairfield Osborn, Charles Robert Knight, Erwin S. Christman



H.F. Osborn's family tree of monkeys, apes, and humans. From Men of the Old Stone Age.


Phylogenetic trees that distanced humans from living apes were not new when Osborn published, though. Taking a cue from the German embryologist Ernst Haeckel, the American paleontologist E.D. Cope suggested that humans had evolved directly from an ancestral stock of lemurs ("without going through the monkeys proper"). Indeed, in his History of Creation Haeckel saw various lemurs as the root stock from which various groups of placental mammals sprang, rodents arising from creatures like the aye-aye, bats from the colugo (known not to be a lemur today, but considered to belong to that group by Haeckel), and humans from large lemurs like the indri. Given that the gap between humans and the earliest fossil primates was exceedingly vast, it was such living forms that some naturalists turned to for clues, and the differences between what we now call strepsirrhine primates and the monkeys suggested a somewhat unique evolutionary path for humans and apes.

Likewise, in 1897 the lecture The Descent of Primates by Dutch zoologist A.A.W. Hubrecht set humans apart from other living primates. Navigating carefully, Hubrecht stated that if a member of the audience suggested it, he would not object to the idea that;

... a direct ancestor of the anthropoids and man, differing from Simiae Catarhinae, Platyrhinae, and Tarsidae, must have existed throughout the Tertiaries, and must have directly sprung from a Mesozoic insectivorous ancestor, small in size, but already more or less erect in posture, provided with a spacious brain cavity ... Now, in suggesting the existence of this unknown intermediate form, you would not be overdrawing the amount which is booked to the credit of scientific speculation in the bank of probability.

It seems that Hubrecht was waiting for any sword hanging above his head to drop when he made this suggestion, for after he broached the topic and was not shouted down he quickly made it his own with a smattering of evidence. Perhaps even stranger to us now, though, was his support for T.H. Huxley's hypothesis that mammals had evolved directly from amphibians and had not gone through a reptilian, shelled-egg-laying phase. (Indeed, one of the most vexing questions I have come across in looking into Victorian science is why Huxley preferred an amphibian ancestor for mammals than a reptilian one, particularly since he studied the "mammal-like reptiles" from South Africa. His position may have had something to do with his enmity towards Richard Owen, who also worked on those fossils, and is explored in Adrian Desmond's Archetypes and Ancestors and Peter Bowler's Life's Splendid Drama.) The supposition that mammals had evolved from reptiles, Hubrecht averred, had for too long hinged on the modern day existence of egg-laying monotreme mammals like the duck-billed platypus. Instead he stated that the placentas of placental mammals showed more similarities to structures in amphibians, thus making the monotremes some kind of aberrant offshoots.

How could these hypothetical relationships be confirmed? Paleontology, Hubrecht said, held the key. Where once the fossil record was seen as being relatively complete, offering almost no transitional forms, a tempting collection of a few intermediate fossil forms here and there hinted that there was much more to be found. The study of fossils did not hold all the answers and studies of living animals provided essential clues, but paleontology offered the ability to test the trees constructed for the ancestry of different groups. Hubrecht urged his Princeton audience to start to work on the questions he raised at once, and he closed his lecture by saying;

I trust that you will kindly account for my readiness in formulating this desire by my confessing that in the last few weeks I have contracted the somewhat awkward habit of believing that the expression of a wish is in this country the surest and shortest way towards the rapid realization of it.

Just as there was no little "Eohomo" around to ride Eohippus during the Eocene, no evidence has ever been recovered to suggest that humans and apes evolved from a distinct, erect Mesozoic ancestor, and the notion that mammals evolved directly from live-bearing amphibians has been out of fashion for so long as to be generally forgotten. Yet Hubrecht's hypotheses for the origin of humans and mammals are just two among many promulgated during the time that evolutionary theory was going through some substantial growing pains. We like to tell stories of progress, of a succession of discoveries making sense of our place in nature (even I have fallen into this trap), yet such an approach largely ignores how science actually works.

In many cases, including the evolution of whales, birds, humans, and early tetrapods, it has only been recently that a large mass of evidence has coalesced to confirm or refute competing hypothesis about origins (and the debates still go on). Indeed, as sure as we may be now, discoveries made in the coming months, years, and decades may very well make us look a bit foolish, but this is the strength of scientific inquiry. There is always something new to discover and debate, and evolution presents perhaps the most sublime riddle of all.

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I also have seen a paper by Hubrecht in which he argued that chordates evolved directly from coelenterates (although he earlier proposed a nemertean origin of chordates). He must have preferred a version of evolution "towards man" that took as few intermediate steps as possible.
By the way, the idea that humans evolved diretly from early tarsioids hung on till the 1940s, with F. Wood-Jones as its last significant proponent.

By Lars Dietz (not verified) on 24 Sep 2008 #permalink

... the evolution of whales to birds to tetrapods to humans...

That's a very interesting line of descent, with the additional virtue of answering the age-old question, "Why are there still monkeys?"!

By Pierce R. Butler (not verified) on 24 Sep 2008 #permalink

Thanks Pierce; I try to keep it interesting, if nothing else. ;) That was poorly worded, though, so thanks for pointing that out. If you think that's something, though, you should see Geoffrey St. Hilaire's conception of the evolution of mammals from marine crocodiles. I don't have it in front of me, and it's not known if he was entirely serious, but it is probably one of the most bizarre lines of descent ever proposed.

Evolution of mammals from marine crocodiles? Where would I find more about that?

Wow, Victorian science was crazy. I ought to be surprised about "mammals-from-amphibians", but considering that John Harvey Kellogg (in defending vegetarianism) quoted a "Professor Elliot" (G. F. Scott Elliot) as claiming that "there was not, so far as we are aware, any carnivorous creature in the Eocene, or one which might have been a serious enemy [to the first primates]", and H. M. Ami as stating that, in Kellogg's words, "all paleontologists agree" that man began to eat meat after the Ice Age wiped out "the great forests of nut trees and wild fruits".

Sorry for the long first comment! This blog is neat, BTW.

By William Miller (not verified) on 24 Sep 2008 #permalink

Thank you for this posting, Brian. It's fascinating to see how much the family tree models have changed in the last 100 years. In both physics and phylogeny there have been amazing changes since the 1890s.

You wrote "...Henry Fairfield Osborn proposed that we shared some unknown common ancestor with gibbons during the Pliocene", but the diagram seems to suggest that Osborn thought humans were more closely related to chimps and gorillas, and the common ancestor was back in the Miocene or even Oligocene, not the Pliocene. Perhaps he stated it differently in his book.

By JJ Anderson (not verified) on 24 Sep 2008 #permalink

... the evolution of mammals from marine crocodiles.

Well, you wouldn't expect us to be descended from those wimp army crocs, wouldja?

By Pierce R. Butler (not verified) on 24 Sep 2008 #permalink

Umm. Sweat glands, not to mention mammary glands-- mammals have vascularized skin. At least superficially this is a similarity betw. mammals and amphibians not shared by reptiles. --> So the inference was that mammals had evolved from something that, though perhaps reptile-like in other ways, still had some of the primitive characteristics that extant reptiles have lost. I'm not sure when it became generally accepted that mammals were descended from "mammal like reptiles," but even this is consistent with the "mamm. from amph." line: after all, the physiological features that inspired the line (e.g. skin structure) aren't preserved in fossils, so it is consistent to hold that mammal like reptiles were more amphibian-like in these regards than are modern reptiles.

Ironically, you can argue that "mamm. from amph" is right by (selective) appeal to modern cladistic ideas. The split between Synapsids and Sauropsids seems to have been earlier than the splits between the ancestors of the extant "reptile" groups. (Certainly before the split between lizards and archosaurs, probably before the split between them and turtles even if turtles turn out not to be diapsids.) So mammals are NOT descended from (crown-group) reptiles. Mammals ARE descended from stem-tetrapods, but then the things we have now learned to call stem-tetrapods were called amphibians until the last few decades.

By Allen Hazen (not verified) on 24 Sep 2008 #permalink

Mammals ARE descended from stem-tetrapods

Really? Stem-amniota, sure, but isn't that a different category which excludes "amphibians" by any definition? (Obviously they're descended from stem-tetrapods in a trivial sense: they're tetrapods.)

I recall reading several years ago (I can no longer remember where) that there are certain aspects of the mammalian circulatory system which are much more easily derived from the amphibian condition than from the reptilian one. This of course refers to living amphibians and reptiles. I guess this raises the possibility (nothing more) that amniota is not monophyletic, or alternatively that hypothetical 'stem amniotes' exhibited amphibian like circulatory systems.

With regard to the synapsids of the Permian and Triassic, we have very little information on their soft biology - and as far as I am aware for the majority of them what little is known (brain morphology etc) seems mostly typical of reptiles.

I read somewhere that Lamarck also derived mammals from crocodiles, except for monotremes which he derived from birds, which were in turn derived from turtles. I'll have to look this up in the original to see if he hasn't been misrepresented.

By Lars Dietz (not verified) on 25 Sep 2008 #permalink

Just checked it. Lamarck really proposed that.

By Lars Dietz (not verified) on 25 Sep 2008 #permalink

Thanks for the comments, everyone.

As for a possible disparity between the diagram and the text, I have seen another sources cite that Osborn thought we had our own pre-human ancestor, distinct from apes, even though I could not explicitly find this in the passages I looked at. I modified the text in the post to make it a little clearer that he thought we shared a common ancestor with all apes, but that generally each line split and went their own way (with the exception of chimpanzees and gorillas, which had a later common ancestor in his arrangement).

The idea that monotreme mammals derived from pre-avian reptile critters loosely connected to marine crocs isn't so far fetched. The platypus hatchling has an egg tooth, which could be viewed as a cause of mammary gland development, milk is immunoprotective secretion from modified apocrine glands associated with hair, a response to getting gently pecked that eventually became suckling. Croc and bird mothers which build nests/burrows and set on them to maintain consistent warmth and protection are in a similar situation but lack hair follicles and apocrines. But where did hair come from? Eyelash scales? Hair cells in the inner ear and (fish) lateral line? It's from deep in the skin, but contains no blood as feathers do. Perhaps it began as armadillo-like scales or plates, glued together with sebum from the associated sebaceous glands? The sweat glands beneath the skin surface are odd too, how they twist around and around, almost resembling intestines or brains, rather than a simple hair-like stalk. Puzzling.