Brian Switek Musings on Evolution, the Fossil Record, and the History of Science

Fossil primate Saadanius provides context for the ancient ape/Old World monkey split

Imagine that there was no primate fossil record. No hominins, no Proconsul, Dryopithecus, no Eosimias, no Darwinius -- nothing. Now, given this dearth of fossil material, you could be excused for systematically organizing primates according to the stark divisions apparent between living species. Our species, while clearly a primate, would seem to stand by itself, and apes would be distinct from monkeys. Likewise, there would be a great division among monkeys - those of the Old World and those of the New - and the various lemurs, lorises, and galagos would fall within their own separate designation. Much like us, tarsiers would seem like another oddball group off by themselves. The terms "human", "ape", and "monkey" would have discrete meanings without any bleeding between categories.

While the basic premise is thankfully false - our knowledge of the primate fossil record expands with each year - primates were often organized in this kind of top-down manner. It was not so long ago that our species, and our species alone, was the sole living member of the Hominidae, and all apes were lumped within the Pongidae. This was the taxonomic signal of what paleoanthropologists believed to be a deep divergence between humans and apes, perhaps as long as 22 million years, but since that time genetic tests and new fossil discoveries have confirmed that we actually belong within the ape group, with our last common ancestor with chimpanzees existing around six million years ago. Expanding this general rule to primates as a whole, by the late 20th century it no longer became profitable to organize the group according to species in existence. The numerous expired branches of the primate family tree had to be taken into account, one in which words like "monkey" and "ape" can refer to more inclusive groups of primates of which we are a small part.

The trouble is that we are still stuck with the baggage of the old "top-down" system. For much of the public, a term like "monkey" identifies a specific grade of primate which cannot also be applied to apes or us - only a monkey can be a monkey. Without the right vocabulary, however, it can be exceedingly difficult to make sense of new fossil primate discoveries, especially since primate evolution is best represented by a wildly branching tree in which many lineages have been entirely eliminated. Such is the case with Saadanius hijazensis, a newly-discovered fossil primate which is causing paleontologists to revise what they thought about the timing of one part of primate evolution.

Known from a partial skull discovered near the Red Sea coast in Saudi Arabia, Saadanius lived in a freshwater mangrove swamp about 29-28 million years ago. As Iyad Zalmout and co-authors state in the introduction of the paper, it is the most substantive discovery of its kind for this part of earth's history - the later part of the Oligocene - but what has caused Saadanius to be catapulted into the headlines is its relationship to other primates. It appears that Saadanius sat close to the what would have been the last common ancestor of living Old World monkeys and apes, and thus provides scientists with some important clues about what the lineage leading up to this great split was like.

In technical terms, Saadanius was a catarrhine primate. Today, catarrhines are represented by Old World monkeys and apes (which together compose what is called the "crown group" of catarrhines), but there were also many lineages of catarrhines in the distant past which became extinct. Saadanius belonged to one of these groups, a catarrhine but not a member of the crown group, and therefore belonged to what is called a "stem group." In other words, Saadanius can be roughly described as a fossil monkey which was closely related to the last common ancestor of Old World monkeys and apes, but was not itself an Old World monkey or ape. (And, appropriately enough, Saadanius is actually derived from the Arabic term saadan used for both monkeys and apes.)

The reason the paleontologists who described Saadanius know this is because of how it compares to early apes, early Old World monkeys, and enigmatic groups which also fall around the crown catarrhine split. On the one hand, Saadanius lacks the conspicuously large canines, frontal sinuses, the broadened front of the palate, and other features which distinguish the earliest apes,  yet, on the other hand, Saadanius has a peculiar condition of the inner ear called a tubular ectotympanic. This feature has only been seen among crown catarrhines before, so if it possesses it, then Saadanius was obviously close to the split. Overall, the skull of Saadanius resembles those of other early catarrhines, but this feature (along with some similarities in the way its face is constructed) place it much closer to crown catarrhines. Had its inner ear not been preserved, or had it only been known from teeth, it probably would have been placed further away from the split.

Significantly, Saadanius may also provide a test of hypotheses about when the earliest Old World monkeys and apes split from each other. On the basis of fossil evidence, the split has often been estimated between 25 and 23 million years ago, but divergence estimates based on genetic data have proposed an even earlier date - between 35 and 29 million years ago. The discovery of Saadanius, so close to the crown catarrhines, falls between these two estimates, putting the divergence between 29 million years ago and the earliest known crown catarrhines from 23-20 million years ago. As always, the discovery of other primates from within this timespan will be needed to sort things out, but Saadanius has provided paleontologists with a key with which to compare some of the traditionally enigmatic catarrhines from the later part of the Oligocene. It wasn't an Old World monkey, and it wasn't an ape, but it provides the context for the origins of both lineages.

Zalmout, I., Sanders, W., MacLatchy, L., Gunnell, G., Al-Mufarreh, Y., Ali, M., Nasser, A., Al-Masari, A., Al-Sobhi, S., Nadhra, A., Matari, A., Wilson, J., & Gingerich, P. (2010). New Oligocene primate from Saudi Arabia and the divergence of apes and Old World monkeys Nature, 466 (7304), 360-364 DOI: 10.1038/nature09094