Hubrecht's upright, insect-eating proto-primate

At the turn of the 20th century evolutionary biologists faced a significant problem. In 1859 Charles Darwin had expounded the mechanism of evolution, and Eugene Dubois' discovery of "Pithecanthropus" (known as Homo erectus today) illustrated that humans had evolved over time, but the broad outline of human evolution was almost entirely a mystery. There was no doubt that we had evolved from some ancient group of primates, but one of the most frustratingly difficult to resolve questions was what our earliest primate ancestors were like.

The Dutch zoologist A.A.W. Hubrecht forwarded his hypothetical view of our early origins in an 1897 lecture The Descent of Primates delivered during Princeton's sesquicentennial celebration. While the present diversity of life presented only a small fraction of all the creatures that ever lived, Hubrecht noted, paleontology had been essential to fleshing out branches of the evolutionary tree that would otherwise be invisible. More than that, fossils sometimes provided "intermediate links" between groups of creatures that today appear to be widely separated.

One such "link" was a small fossil primate called Anaptomorphus homunculus by E.D. Cope. Today we know Anaptomorphus is an omomyid, a small primate that lived during the Eocene in North America, but (as Hubrecht noted) Cope thought it had a special relationship to humans. It would have looked something like a living tarsier, the wide-eyed primates of modern Southeast Asia, and by studying living tarsiers Hubrecht thought he could reconstruct the biology of Anaptomorphus.

Part of the reason Hubrecht focused on tarsiers was that they were distinct from other "primitive" primates like lemurs, with which they had traditionally been grouped. Hubrecht thought that tarsiers were too different from lemurs to simply be lumped in with them, though, and the "wide divergence" between tarsiers and lemurs was driven home by differences in tooth structure and reproductive biology. Generally speaking, tarsiers seemed more similar to insectivorous mammals and even humans than did lemurs. More striking than that, tarsiers seemed to be more closely related to monkeys, with lemurs representing a side-branch of primate evolution more distantly related to monkeys and apes (anthropoid primates).

How deep in time did this divergence go? At this point Hubrecht returned to Anaptomorphus. It was not implausible that such a tarsier-like primate would be very similar to living tarsiers, and if this was true then lemurs must have diverged from other primates at some earlier time. Lemurs could not be counted as our ancestors, but neither did Hubrecht think that we were very closely related to monkeys.

Even though Anaptomorphus was tarsier-like, Hubrecht seemed to favor it as more of a last common ancestor between the lineage that led to monkeys and the one that led to apes. This was based upon Hubrecht's appraisal of Anaptomorphus as having more in common with apes than with monkeys, and Hubrecht reinforced this hypothesis through embryology. Hubrecht thought that aspects of the placenta and other facets of reproductive biology in insectivores like hedgehogs better "anticipated" the same organs in humans than in monkeys. If this connection was real, he argued, then it was likely that humans were actually not very closely related to monkeys at all. Instead he argued 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.

In other words, as fantastic as it might have seemed, Hubrecht was arguing that after the split with lemurs there was another major evolutionary split, with humans and apes on one side and tarsiers and monkeys on the other. We had not passed through a "monkey stage" in our evolution but had evolved directly from a small insect-eating mammal that was already standing upright during the waning dominance of the dinosaurs. Such would be the appearance of the "intermediate link" between primates and earlier mammals when found.

The idea of an upright, proto-primate was interesting, but Hubrecht was faced with a substantial dilemma. He based much of his case upon embryology and the biology of living creatures, or features that could not be observed in fossil animals. What Hubrecht had inferred hinged on as yet undiscovered fossils, but even without these finds progress towards understanding primate relationships could still be made. The embryology of the New World monkeys, for instance, had not yet been studied in detail, and a more complete knowledge of their development would allow for better resolution of primate relationships. Hubrecht implored his American colleagues to begin such an scientific endeavor, 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.

Ultimately Hubrecht's vision of a tiny, upright, bug-chewing ape ancestor was cast off. He was correct about the close relationship between tarsiers and anthropoids, but there was no separate ancestry for apes and monkeys. Why, then, should we bother to consider his discarded ideas over one hundred years later?

The ongoing fracas over "Ida" reminded me of Hubrecht's lecture. Scientists have long been in search of the extinct creatures that root us in the mammalian family tree, and debates about our early primate ancestors will be ongoing. We do have a lot more information than Hubrecht did when he delivered his lecture over a century ago, but there are still many questions that require resolution. Ida, alone, will not solve everything. There is still much to do, and even if the mass media does not pick up on them I am glad that scientific investigations about our early primate relatives will continue.