March of the mastodons

Regular readers of this blog are well aware that the "March of Progress", a depiction of the single-file evolution of humans from an ape ancestor, is a biological bugbear that refuses to go away. Even though the Great Chain of Being ceased to be useful in explaining the natural world centuries ago vestiges of it still remain in illustrations that depict evolution as "onward and upward." We have long known that evolution is a branching process yet the straight-line version is frustratingly difficult to dig out.

I was reminded of this while searching for something interesting to say about the extinct elephant-like creature called Gomphotherium. My efforts to find some interesting tidbit was initially confused by the fact that, for reasons as yet unknown to me, E.D. Cope applied the name Gomphotherium to a genus of North American camel in 1886 even though the name had already been used for the proboscid decades earlier. This mistake was corrected by 1901 at the latest (even though R.S. Lull still used the old name in his textbook Organic Evolution in 1917). As far as I can tell many of the camel "Gomphotherium" fossils turned out to belong to the genus Protomeryx, although this is now itself a nomen dubium.

Interesting, but not really what I was looking for. I decided to keep pushing through the taxonomic muddle to find some long-forgotten hypothesis about the elephantine Gomphotherium and I was surprised to find a 1914 University Studies of the University of Nebraska paper by E.H. Barbour
featuring an elephantine March of Progress. I had seen similar illustrations for humans, horses, brontotheres, and even a modern example featuring whales, but I had never seen the elephant version before. Just as the understanding of horse evolution seemed all-but-complete in the early 20th century elephants, too, seemed to have an exceptionally complete fossil record. Barbour wrote;

The genealogy of this group is now so well known to naturalists, that it is interesting to note in the writings of Cope and others of twenty-five years ago, that the intermediate proboscideans are entirely lost, and the phylogeny of the order absolutely unknown. As a reward of zeal, the genetic gaps are being filled so rapidly, that ultimately knowledge of the history of the Proboscidea must be as well known as that of the Equidae.

This understanding was translated into an illustration of a straight line of elephant evolution, almost (but not quite) resembling a hypothetical growth series for the upper-most rung on the ladder, the mammoth. Particularly telling is the place of the mastodon (Mammut americanum) below the mammoth (Mammuthus). Mammoths and mastodons were contemporaries and should have probably been placed next to each other. Instead the mammoth, generally more popular than its mastodon cousin, occupied the top rung.

Even though we now know elephant evolution to be a bit more bushy than Barbour did he did recognize that there was some branching within it. In a phylogenetic tree included in the paper Barbour placed Deinotherium and "Dibelodon" (now Rhynchotherium) on side branches. Papers on horse evolution followed a similar pattern. There was often a detailed phylogeny showing a branching bush of diversity included with another that showed the straight-line evolution towards modern forms at the same time. It was the latter illustrations, and their counterparts in museum displays, that made the most impact and remain with us to this day.

I have to wonder how much the March of Progress-type illustrations were influenced by the acceptance of non-Darwinian evolutionary mechanisms by paleontologists in the early 20th century. The idea that organisms were being driven toward a particular evolutionary goal was a popular one and this certainly influenced how paleontologists interpreted the importance of side-branches. Today we appreciate how natural selection creates diversity but when orthogenesis was the accepted framework evolutionary side-branches* simply become "losers in life's race."

Lineages that were closely related to the "main line" of evolution but became extinct were often considered to be following a parallel evolutionary path that, if they had survived, would have driven them to the same evolutionary goal. This is especially important in considerations of human evolution as it may be key to why Australopithecus was not appreciated as an early human until the middle of the 20th century. (For more on this see Tom Gundling's First in Line: Tracing Our Ape Ancestry).

I honestly do not know what evolutionary mechanism Barbour preferred, but the illustrations in his paper show the tension between reflecting the actual pattern of evolution and connecting the past to the present. It is the same with retelling the history of science. It is often more expedient to quickly review the simplified version of how this or that scholar gave rise to an entire discipline than to review the complicated nature of how scientific understanding changes. It is certainly not a simple accumulation of knowledge over time. Narratives or phylogenies inevitably are pruned down so that the continuity between the past and present becomes more important than the bigger picture of intellectual or evolutionary change. Both concepts need to be understood, but it seems to me that we will continue to see dueling illustrations of evolution for some time to come.