Palaeontology

Combing the primate record

Fossils of bushbabies and lorises reported from deposits of the Fayum Depression in Egypt extend the known record for this group of primates from 20 million years to approximately 40 million years ago.

Primate evolution attracts special interest because of its direct relevance to human origins. The fossil record remains extremely sparse, however, and that is particularly true of a group called the strepsirrhines, which comprises the bushbabies and lorises — together, the lorisiforms — and the lemurs. Part of that picture now changes radically with the report, by Seiffert, Simons and Attia on page 421 of this issue1, of plausible early members of the bushbaby and loris lineages. The strepsirrhines are the 'sister group' of the haplorhines — the tarsiers and higher primates, which include ourselves. The relationships among the main groups of living primates are shown in Fig. 1.

Figure 1: A simplified primate evolutionary tree.
figure1

The two main groups of living primates, the strepsirrhines and haplorhines, are shown, and the new fossil forms (Saharagalago and Karanisia) described by Seiffert and colleagues1 are included. These specimens double the known age of the lineages leading to bushbabies and lorises (together, the lorisiforms), and increase the number of known lorisiform genera by half. The previously known genera are Komba, Progalago and Mioeuoticus from the early Miocene of East Africa, and Nycticeboides from the late Miocene of Pakistan. The only known fossil form potentially related to the other strepsirrhine group, the lemurs, is Bugtilemur from the early Oligocene of Pakistan. A direct reading of the fossil record indicates that primates began to diversify just after the Cretaceous/Tertiary (K/T) boundary. But, given the incompleteness of the fossil record, diversification may have occurred at least 20 million years before the K/T boundary, as indicated by the shadow tree. (Primate icons drawn by Lucrezia Beerli-Bieler.)

The lemurs are found only in Madagascar, and consist of 15 modern genera and 54 species2. Their palaeontological record is virtually zero. The record of lorisiforms, a smaller assemblage of 8 modern genera and 32 species2, was scarcely better: until now, the earliest known fossil representatives were three early Miocene genera from Kenya (Komba, Mioeuoticus and Progalago), up to 20 million years old3. Seiffert and colleagues' discovery of isolated teeth and jaw fragments of two new genera, Karanisia and Saharagalago, from late middle Eocene deposits of Egypt, improves matters considerably. Both forms share unique dental features with modern lorisiforms, notably a prominent fourth cusp (hypocone) on each upper molar, contributing to a distinctive concave posterior tooth margin. In one fell swoop, Karanisia and Saharagalago, dated at between 37 and 41 million years old, have doubled the age of the known fossil record for lorisiform primates and illuminate the African diversification of lorisiforms.

An overall assessment of morphological, chromosomal and molecular evidence supports the view that the strepsirrhines and haplorhines diverged early in primate evolution4. Morphologically, strepsirrhines are clearly distinguished by their retention of various primitive features, such as the rhinarium (a naked area of moist skin surrounding the nostrils), and by a uniquely derived toothcomb. This structure is used both for feeding (for instance, scooping pulp from fruit and scraping gum from trees) and for grooming the fur, and is formed by the canines and incisors in the lower jaw (six teeth in all). These teeth are elongated and bilaterally flattened into tines, and their narrow crowns angle sharply forwards from the roots, almost horizontally. Hairs dragged between these tooth crowns during grooming leave conspicuous grooves, clearly recognizable on fossilized teeth5. A convincing indication that Karanisia is, indeed, a strepsirrhine is that a lower-jaw fragment shows typically compressed tooth sockets for the narrow roots of the canine and incisors, and the crown of an isolated canine shows tiny lateral grooves worn by the passage of hair. This neatly confirms the prediction that the toothcomb was present in early strepsirrhines, and also provides direct fossil evidence of the occurrence of grooming behaviour 40 million years ago.

Soon after the initial split between the toothcombed strepsirrhines and the haplorhines, there was a divergence between the lemurs and the Afro-Asian lorisiforms4,6,7. The lorisiform lineage then divided into bushbabies and lorises (Fig. 2), a subdivision that may also be documented by the new fossils. Molars of Saharagalago strikingly resemble those of modern bushbabies, whereas those of Karanisia are generally similar to loris molars, particularly those of the African angwantibo (Arctocebus). However, one of the most striking differences between modern bushbabies and lorises involves locomotor adaptations: lorises typically climb sluggishly (reflecting their very low basal metabolic rates), whereas bushbabies commonly leap actively and show conspicuous elongation of the tarsal bones of the hindlimb. Isolated tarsals from Progalago show moderate elongation, but in the absence of limb-bone fossils of Saharagalago or Karanisia, we cannot say whether these earlier forms already showed the expected divergence in locomotion. Modern bushbabies also clearly differ from lorises in possessing molar-like posterior premolars. The discovery of similar teeth in Saharagalago would provide a valuable additional test of the inferred relationship to modern bushbabies.

Figure 2
figure2

LEFT: CLEM HAAGNER; GALLO IMAGES/CORBIS; RIGHT: K. A. I. NEKARIS

Lorisiforms here and now — a bushbaby (left) and a loris.

Saharagalago and Karanisia are the latest discoveries in four decades of fossil-hunting by Elwyn Simons and colleagues in the Fayum Depression. The Fayum site, on the eastern margin of the Sahara in Egypt, contains extensive sediments spanning the late Eocene and the early Oligocene. It provides the best known record for the early evolution of modern mammals in Africa. This single site has yielded a remarkable diversity of primate fossils, most being of higher primates, although previous finds include Afrotarsius (possibly related to modern tarsiers) and Plesiopithecus (an aberrant early strepsirrhine offshoot).

More broadly, Seiffert and colleagues1 conclude that the new finds are compatible with a date of 50–53 million years ago for the last common ancestor of strepsirrhines, with Afro-Arabia being the location of that ancestor. But this conclusion stems from a direct reading of an inadequate fossil record. We still have no fossil primates from sub-Saharan Africa before the Miocene, and the fossil record for Madagascar lemurs remains nil. My own alternative interpretation stems from statistical considerations8 indicating that such gaps in the primate fossil record have led to a serious underestimation of divergence times. Furthermore, molecular phylogenies for placental mammals9 have revealed a group of endemic African mammals (Afrotheria) that does not include the primates, suggesting that primates originated elsewhere. Various molecular trees also indicate that primates originated much earlier than generally accepted, about 90 million years ago. One possibility is that strepsirrhines originally inhabited Indo-Madagascar, rather than Africa, and that lemurs became isolated when Madagascar separated from India. Subsequently, lorises could have migrated to Africa after India collided with Asia, reaching Africa during the Eocene.

Interestingly, the only known fossil primate with direct affinities to lemurs (Bugtilemur; Fig. 1) was reported from Oligocene deposits in Pakistan10: its dentition closely resembles that of modern dwarf lemurs. Bugtilemur raises more questions than it answers, however, and shows that we still have much to learn about the early evolution of our primate relatives.

References

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Correspondence to Robert D. Martin.

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