The horse as an evolutionary paradox

When Charles Darwin published On the Origin of Species by Means of Natural Selection in 1859 he faced a substantial problem. His evolutionary mechanism predicted that the fossil record would contain finely-graded transitions revealing what G.G. Simpson would later term the "tempo and mode" of evolution, yet such transitional creatures proved elusive. This was a major problem for paleontologists who believed that studies of European geology represented an almost complete picture of Deep Time that had already been examined carefully. If the fossil evidence for Darwin's theory had not yet been found then there was little chance it would be in the future.

Darwin, of course, did not agree. Following the lead of his friend and mentor Charles Lyell, Darwin argued that the fossil record was not as complete or well-studied as was supposed. The discovery of the skeleton of the dinosaur-like bird Archaeopteryx in 1861 alone showed that the fossil record held many secrets left to be revealed. Other less spectacular "intermediate" fossils came to light or were reinterpreted in an evolutionary light, and in 1870 T.H. Huxley was able to propose at least three evolutionary transitions among vertebrates supported by fossil evidence.

Huxley knew well enough that two of the transitions, from terrestrial carnivores to whales and reptiles to birds, could not be represented in a real ancestor-descendant series. Instead Huxley said that the fossils in these transitions represented the form we might expect "straight line" ancestors and descendants to take. The small dinosaur Compsognathus, for instance, was not a bird ancestor but a dinosaur representative of the kind of animal from which birds evolved.

Horses were another matter. The work of paleontologists like Albert Gaudry and Vladimir Kowalevsky had illustrated the linear descent of modern horses (Equus) from multi-toed ancestors like Anchitherium and Hipparion. (Palaeotherium seemed like a good candidate for the evolutionary starting point for horses, but this was debatable.) This was a perfect graded evolutionary chain that appeared to confirm Europe as the birthplace of horses.

This hypothesis was quickly scuttled by the collection of numerous fossil horses from the American west, particularly by paleontologist O.C. Marsh. Horses had evolved in North America, not Europe, and the horses that formed the backbone of the lineages outlined by Gaudry, Kowalevsky, and Huxley represented "invasions" of types that had evolved in North America first. (See my review of The People's Peking Man for some brief thoughts on fossils and national pride.)

Despite the diversity of horse types found in North America, however, they were still organized in a progressive march from small, multi-toed browsers (Eohippus) to large, one-toed grazers (Equus). The branching nature of horse evolution was recognized in technical papers but most popular discussions of horse evolution, from textbooks to museum displays, drove home the progressive narrative. This series presented undeniable evidence for the evolution of an animal familiar to all, but by what mechanism did the modern horse evolve?

As I have mentioned previously many late 19th and early 20th century paleontologists were reluctant to accept natural selection as the primary mechanism of evolution. Competing mechanisms like neo-Lamarckism and orthogenesis were more popular. This was at least partially due to the sparse nature of the fossil record; there were gaps between groups that seemed to require some sort of jump or mechanism more able to produce rapid change than selection on variation. These competing mechanisms were also more easily squared with religious views, for evolution seemed less threatening if it had a direction (i.e. fore-ordained end points various lineages were striving towards).

The linear march of horse evolution was a perfect example of the belief in a directional kind of evolution. The descendants of little Eohippus were evolving towards the Equus type and those that had died out were simply unsuccessful experiments that, if they survived, would have likewise evolved into a large, high-toothed, one-toed horse. This type of evolution could be compared to the development of an individual over a lifetime; the changes in morphology that were to occur were pre-determined. (Orthogenesis was also applied to other groups like elephants and humans. Likewise, some scientists thought that species became extinct as a result of senescence, meaning that the lineage naturally died due to a loss of evolutionary vigor.)

The problem was that all the major changes in horses that paleontologists fixated upon (size, tooth height/type, and number of toes) could be understood as adaptations to changing environments. Either horse evolution was truly being driven by natural selection or directional forces were mimicking true adaptation. This paradox was pointed out by T.D.A. Cockerell in his 1920 textbook Zoology;

Naturalists ... have sometimes postulated what they called orthogenesis, the first part of the word meaning "straight" or "regular," as in orthodox. This implies that evolution follows a predetermined path, which was laid out for it in the beginning. Thus, the horse group was to increase in size, decrease the number of its toes, etc. It actually behaved as if following out a program planned in advance. The idea is not inherently absurd, since this is the course of individual development ; and it may well be imagined that there is something in the nature of a particular kind of protoplasm, that will lead it to vary in a certain direction. Indeed, we know that it does not vary in all directions ; thus we cannot get a genuinely blue rose. It is to be noted, however, that the evolution of the horse group is also strictly along the lines of adaptation. The climate became cooler and drier; the animal became an inhabitant of the plains. The solid hoof is adapted for running on hard ground, for receiving the impact of the heavier body; also for kicking the carnivorous enemies which had in the meanwhile evolved to prey upon the horse. The long-crowned, hard teeth are adapted for feeding on the vegetation to be found in open, dry places, and what might be regarded as a difficulty has been so completely overcome that the animal now- needs the type of food for which it is specially fitted. The whole history is one of adjustment to conditions, and the evolutionary process could not have taken place in the Eohippus environment, for the simple reason that the changes would all have been detrimental, leading eventually to extinction.

It is not explicit, but in this passage Cockerell proposes a compromise between orthogenesis and adpatation. An organism may contain variations to allow the next stage of development to take place but the conditions have to be right for those variation to be viable. If Eohippus had grazing dentition, then, according to Cockerell's model it would die. There was a barrier to further development that had to be removed. Thus we have an image of lineages striving "upwards" but perhaps blocked by environmental conditions. Only when the environment changes in the right way does the organism become "released" to evolve to the next stage.

Orthogenesis was ultimately crushed by the development of the modern evolutionary synthesis during the 1940's and 1950's. (G.G. Simpson was instrumental in extirpating it from paleontology.) The presence of a natural force driving organisms to particular ends was an inference that had no solid evidence to support it. Rather than being a true evolutionary mechanism it was more of a framework for understanding the pattern of evolution over millions of years. (For a recent summary of our present understanding of horse evolution see this article by Bruce MacFadden.)

Even today the evolution of the horse is often presented in a straight-line fashion with little attention to the previous diversity of the group. Why should this be so? There is a sort of "pull of the recent" in science where we wish to know the linear story of how we got to where we are now.* This is the desire that has constructed faulty phylogenies and promulgated "textbook cardboard" versions of the history of science. It seems that we can readily appreciate the diversity of groups that have left no living descendants, but we are more comfortable telling linear stories to explain the presence of living groups. It may indeed be possible to do this but it also requires some pruning and pushing extinct branches off to the sidelines. This is why we tip our hats to robust australopithecines in summaries of human evolution but rarely discuss them in detail.

*[I have borrowed this phrase from paleontology. Its regular usage refers to the fact that the fossil record is generally more complete the closer it is to the present.]

It is unfortunate, and frustrating, that we appear to be stuck with images of evolution that imply progress. We are still grappling with paradoxical images that are supposed to reveal adaptation but more strongly imply some kind of predestined direction. I was reminded of this last night when I picked up Stephen Jay Gould's 1989 book Wonderful Life. Gould opens with a discussion of images of progress in evolution and the strange relationship between scientists and illustration. One would think that scientists would think carefully about the images they use, particularly in paleontology where restorations and diagrams are so often used, but this is not necessarily so. Tradition seems to bind us to particular modes of illustration even after they have become outdated.

Even 20 years after Wonderful Life was published I still see images of evolution that infer progress (especially the erroneous trajectory from chimpanzee to Australopithecus afarensis to Homo sapiens). Are the traditional modes of illustration truly the best, or are we just unimaginative? I am leaning more towards the latter, but I suspect it will always be a problem. There seems to be a continual fascination with how the present world came to be as it is; species or entire lineages that have been lost along the way are thus seem of little relevance. This is absolutely untrue, but it is hard to fight the pull of the recent.