Science Literature

Darwinius masillae is the magnificently preserved Ida, the "eighth wonder of the world" unveiled earlier this year: "our Mona Lisa" and an evolutionary "Rosetta Stone". Afradapis is a newly discovered adapoid from Egypt, known from fossilised jaws and teeth. The controversies surrounding Ida have been the subject of comment here and here. A newly published cladistic analysis of 360 morphological features found in 117 living and extinct primates comes down on the side of Ida being more closely related to lemurs and lorises rather than ancestral to anthropoids.

Darwinius was proposed to be on the right branch - an ancestor of apes and humans - but the new study puts it as a dead end on the left branch (Source here)

One of the criticisms of the original report of Darwinius is that the authors did not provide a comprehensive cladistic analysis, but only referred to anthropoid-like characters. That analysis is still not forthcoming, but a new paper by Seiffert and colleagues considers dentition and jaw morphological features for a comprehensive set of primates, including Darwinius. The claim for anthropoid-like characters is put in a new light, because so many adapiform animals (ancestors of lemurs and lorises) have them.

"It has long been known that some adapiform lineages evolved derived morphological features that are also seen in living and extinct anthropoids (for example, fused mandibular symphyses, upper canines with mesial grooves, enlarged and spatulate upper and lower incisors, short and tall rostra). The phylogenetic significance of these features has been a source of ongoing debate for decades."

Their significant finding is that Afradapis (their newly reported fossil species - an undisputed adapiform) has numerous anthropoid-like characters. This leads the authors to conclude that evolutionary convergences are in plentiful supply.

"Of all known fossil prosimians (including Darwinius), Afradapis provides perhaps the most detailed examples of derived anthropoidlike adaptations in its dental and mandibular morphology. As is the case for many of the morphological features that some have argued link adapiforms to anthropoids, however, the anthropoid-like features of Afradapis (fused mandibular symphysis with transverse torus, deep mandibular corpus, deep masseteric fossa, large upper molar hypocones, absence of P2/2 and presence of an enlarged P3 with a honing facet for the upper canine) are not present in the most primitive undoubted fossil anthropoids, such as Biretia and Proteopithecus, indicating that the features are likely to have been acquired through convergent evolution."

The implication, then is that the Darwinius team have been misled by characters that have turned out not to be diagnostic of anthropoid affinities. This conclusion has been picked up and discussed by many commentators, such as Gibbons:

"When they scored Ida and Afradapis against those other primates, Seiffert and colleagues found that adapids do share some traits with anthropoids, such as the loss of a third upper and lower premolar. But these traits evolved more than once among primates, the team reports tomorrow in Nature. They are the result of convergent evolution, which is the acquisition of the same biological trait in unrelated lineages - and, thus, do not indicate inheritance of the trait from a shared ancestor."

Several significant consequences follow from this research. Not least is the reminder that tracking human ancestors by identifying missing links is an exercise fraught with methodological difficulties. As has been previously noted, human evolution data can be likened to a pointillist painting. Like a pointillist painting, evolution is only apparent from a distant vantage point. Close up, we see masses of data, but no coherent picture.

Another issue to address concerns convergent evolution and the problems this phenomenon creates for cladistic analyses. How do we know what characters are primitive and what are derived? Here is an opportunity for human interpretation to be concealed behind a scientific analysis. The extent to which convergences can be invoked raises suspicion in the minds of some:

"One of the researchers who studied Ida, however, responds that Ida and Afradapis look more like the group that gave rise to anthropoids than the group that gave rise to lemurs and lorises - and that there are too many traits to dismiss as convergent evolution. "The complete convergence postulated for Afradapis seems implausible to me," says paleontologist Philip Gingerich of the University of Michigan, Ann Arbor."

The best outcome of all this is for scientists to demonstrate more humility in handling data. So many seem to grasp at some data and brandish 'evidence' as though it provides a definitive answer to controversy. But this is not how data should be handled in research. Data needs to be interpreted and history shows that there is always more than one way of interpreting it. If we can all adopt a 'multiple working hypotheses' approach when using the scientific method, it will be progress indeed.

Convergent evolution of anthropoid-like adaptations in Eocene adapiform primates
Erik R. Seiffert, Jonathan M. G. Perry, Elwyn L. Simons & Doug M. Boyer
Nature 461, 1118-1121 (22 October 2009) | doi:10.1038/nature08429

Adapiform or 'adapoid' primates first appear in the fossil record in the earliest Eocene epoch (~55 million years (Myr) ago), and were common components of Palaeogene primate communities in Europe, Asia and North America1. Adapiforms are commonly referred to as the 'lemur-like' primates of the Eocene epoch, and recent phylogenetic analyses have placed adapiforms as stem members of Strepsirrhini, a primate suborder whose crown clade includes lemurs, lorises and galagos. An alternative view is that adapiforms are stem anthropoids. This debate has recently been rekindled by the description of a largely complete skeleton of the adapiform Darwinius, from the middle Eocene of Europe, which has been widely publicised as an important 'link' in the early evolution of Anthropoidea. Here we describe the complete dentition and jaw of a large-bodied adapiform (Afradapis gen. nov.) from the earliest late Eocene of Egypt (~37 Myr ago) that exhibits a striking series of derived dental and gnathic features that also occur in younger anthropoid primates [. . .] The specialized morphological features that these adapiforms share with anthropoids are therefore most parsimoniously interpreted as evolutionary convergences. [. . .]

See also:

Gibbons, A., New Primate Fossil Poses Further Challenge to Ida, ScienceNOW Daily News (21 October 2009)

Dalton, R., Fossil primate challenges Ida's place, (21 October 2009), 461, 1040 | doi:10.1038/4611040a