March 1997 - Bridging the GAP - Page 6

TAXONOMY

What’s in a Name?

Carolus Linnaeus, Letter to J. G. Gmelin, February 14, 1747

Traditional taxonomy aims to reflect the evolutionary histories of species, but it does so by a rather subjective evaluation of a range of anatomical and chemical features based on fossils and extant species. Though widely accepted, it has nonetheless been criticized for giving differential weighting to particular characteristics, because of the individual interpretation of the person doing the classifying. This is why, perhaps surprisingly, there is currently no agreed standard for classifying species into higher taxonomic categories (e.g. genus, family).

Under such a classification system, humans are placed in the same Family as chimpanzees, bonobos, orang-utans and gorillas (Hominidae), but are separated out at the Sub-Family level, with humans, chimps and bonobos, and gorillas being placed in Homininae and orang-utans in Ponginae. Further separation occurs at the Genus level, with humans, chimps, bonobos and gorillas being classified as Homo, Pan and Gorilla respectively.

Call for Objective Standards

Simon Easteal, Head of the Human Genetics Group at the Australian National University in Canberra, believes that classification should be based on a more objective, uniform standard - namely, molecular distance (i.e. differences between species in terms of their DNA and protein). Traditional taxonomy is increasingly taking this into account (up until relatively recently humans and the other great apes belonged to different families), but still relies heavily on other, less objective, features (the current separation at genus level is largely based on such features as brain size, upright posture, absence of body hair etc. - these are real differences, but the importance or weight attached to them really depends on your point of view.)

If Easteal’s standard were to be adopted completely, then not only would we all be classified in the same sub-family, but at least the chimpanzees, bonobos and gorillas would be placed in the same genus as humans (with orang-utans remaining in Pongo ). Considering the extraordinarily small molecular differences between us (our DNA differs from orangs by 3.6%, gorillas 2.2%, and from chimps and bonobos by a mere 1.6%), it is hard to see why we’ve been separated for so long.

Further, because the genus Homo was named before the other African ape genera, scientific protocol dictates that this is the name that should be used. What this actually means is that there would no longer be just one extant representative of Homo , but four: Homo troglodytes (common chimp), Homo paniscus (bonobo or pygmy chimp), Homo gorilla (gorilla) and Homo sapiens (human) (a view also reached by Jared Diamond in his work The Rise and Fall of the Third Chimpanzee - an extract from which is reproduced in The Great Ape Project).

Recent Divergence

In addition to voicing support for this taxonomic reshuffle, Easteal has suggested that divergence times (i.e. when new species evolve) have been largely overestimated, and outlines what such findings mean for the fossil record. Current thought has it that the so-called ‘molecular clock’ (the rate at which DNA changes over time) ticks at a different speed depending on what species you happen to be. Specifically, it is thought that the rate of change in humans is slower than that of other mammals, including our fellow great apes. For example, when humans and chimps diverged, their DNA changed independently, with the rate of human change being slower than the chimp’s. This means that their divergence time is considerably earlier than would otherwise be expected, based on a 1.6% genetic difference. Easteal has shown, however, (through a procedure known as the ‘relative rate test’) that the rate of change is in fact the same for all species (at least amongst mammals). Hence, humans should be seen as having diverged from orangs 8-9 million years ago, from gorillas 5 million years ago, and chimps and bonobos 3.6-4 million years ago. As well as setting a cat amongst the pigeons in terms of the fossil record, such an evolutionary shake-up serves to highlight further the close connections between humans and the other great apes.

Bipedalism Not Unique to Humans?

The other consequence of such findings is that the divergence of chimps and humans actually post-dates both Australopithecus afarensis and the recently discovered A. ramidus. This means that these may be the common ancestors of chimps and humans, rather than two species well and truly on the human evolutionary path. And, further, it suggests that bipedalism (so often held up as the exclusive preserve of human evolution) may in fact have been present in one or both of these common ancestors, with chimpanzees then evolving from that state to their present mode of locomotion (i.e. they returned to a largely arboreal existence).

If Easteal’s revision gains the support of the scientific community, then not only would we all be placed in the same family, but the chimps, bonobos and gorillas (at least) would become members of the genus Homo. This would not only represent a radical shift in scientific thinking, but force us all to rethink the way we treat our fellow great apes.

§§§

Geneticists Professor David Penny and Dr Elizabeth Watson of Massey University, New Zealand, both founding members of GAP New Zealand, are working with Dr Easteal on this. In 1989, Elizabeth Watson conducted research that looked at ways of resolving the trichotomy between humans, chimpanzees and gorillas using the pseudogene eta-globin. We hope to have more details in a later newsletter.


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