In search of "Eohomo"

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.

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

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.