Laelaps

March of the mastodons

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The skull of Gomphotherium, from Barbour’s paper.

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.

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Barbour’s illustration of elephant evolution.

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.

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Barbour’s elephant phylogeny.

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.

Comments

  1. #1 Raymond Minton
    February 25, 2009

    Lots of people seem to be stuck on the straight-line , upward progressive model of evolution, whether it be for horses, humans, or, in this case, proboscideans (I’ve seen the early, semi-aquatic Moeritherium described more than once as an ancestor of modern elephants when it was actually a side branch, not a direct ancestor.) A look at the proboscideans, living and extinct, also reveals how widespread and diverse this group once was (reaching it’s apex during the Pleistocene) and how impoverished a group it is now, with few representatives that have been, to one extent or another, threatened by humans.

  2. #2 Laelaps
    February 26, 2009

    Raymond; Right on. The thought just occurred to me to gather all the “March” images I know of in one place, but I just how too many other projects going right now to do so! I just wish that modern biologists would think a little more carefully about the imagery they use when depicting evolution (and I have indeed been agonizing over the illustrations I am planning for my own work).

  3. #3 Corax
    February 26, 2009

    I recall as a kid in the seventies seeing quite a few of these. In addition to humans, horses and elephants, I recall camels, cats and dogs as well as several other less ‘detailed’ ones.

    These all turned out to be wrong – as well as ‘simplifying’ trees to lines, they also contained taxa that had nothing to do with the extant taxa they were shown as precursors to.

    My recollection is that the dog line included some dubious generic basal carnivorans, amphicyonids and hemicyonines as well as bona fide Canidae. The cat line included nimravids and a creodont. The camel one was at least mostly animals still considered camelids, though a few of the early ones were somewhat dodgy.

    As I noted before, the problem isn’t just that they simplified trees to lines, rather it seems that where there wasn’t a convincing intermediary known they coopted whatever was available. I can only assume that this was done in good faith with the intention of presenting a positive convincing case for the evolution of the groups concerned.

    It certainly was impressive to see them as a kid, but as one by one they were shown to be wrong, it rather left a bitter taste, and I am now cynical about any tree/line I see. I guess now as an adult and a scientist, I now know that these were PR not science.

    As a personal observation (possibly controversial) I think the Transitional Vertebrate Fossils FAQ pages of Talk Origins seem to make the same sort of mistakes, some of the transitional fossils seem to be slotted in to fill gaps even though they are very poorly known or only tenuously connected.

    On a more positive note, they were great art, and I would love to see a collection of these put together.