What restrained the chimpanzees?

Working on a paper about early hominid lineage diversity, Milford has pointed out a sticking point in consideration of niche breadth in early hominids. The problem is that chimpanzees have never been found in East African Pliocene. Hominids occupied a boundary woodland between savanna and forest habitats. We can presume that they would have been restricted from fully forested environments by apes; namely, the ancestors of chimpanzees and gorillas. But we don't have the fossil remains to suggest that hominids weren't in forests, or that chimpanzees and gorillas were ever competing with them at a forest-woodland boundary.

A possible alternative view is that the phylogenetic divergence between hominids and chimpanzees was initially a geographic difference, resulting from isolation. The classic story is Yves Coppens' "East Side Story," which attributes the geographic isolation of hominids in East Africa to the initiation of the Great Rift Valley, which today separates the rain forest of central Africa from the lakes and uplands occupied in ancient times by hominids.

But there are some problems with this scenario also. Notably, Pliocene hominids are now known from West Africa. Assuming that Sahelanthropus is an ancient ape, there is still the Bahr el Ghazal mandible to show that early hominids were dispersing west of the rift. And today chimpanzees live in the open woodland and lake boundary habitats in Uganda and Tanzania that we imagine early hominids were living in. If chimpanzees can live there now (and humans don't--or at least haven't historically) then why couldn't ancient chimpanzees have lived there in the Pliocene?

I am inclined toward an alternative interpretation. The reason there weren't chimpanzees in East Africa in the Pliocene is not that they were stuck in the West African rain forests. The reason is instead that there weren't any chimpanzees. My hypothesis is that the modern chimpanzee is very different from its Pliocene ancestors.

Today, chimpanzees are broad dietary generalists, including meat, fruit and leaves in large proportions in their diets. They are effective terrestrial quadrupeds, and range widely into savanna at their easternmost extent. And perhaps most importantly, they face no significant predation from large carnivores. In their current form, chimpanzees would almost certainly outcompete the earliest hominids in their woodland habitat. Chimpanzees are effective on the ground, they climb much better than early hominids could have done, and they appear to have developed effective strategies to deal with predation as well as other chimpanzee groups. The only thing constraining chimpanzees today is humans.

But molecular evidence has begun to tell an interesting story about chimpanzee origins. Apparently chimpanzees and bonobos diverged sometime between 2 million and 800,000 years ago, while living subspecies of chimpanzees diverged less than one million, and possibly only 500,000 years ago. In other words, today's Pan is a Pleistocene genus.

Interestingly, eastern chimpanzees (P. troglodytes schweinfurthii) have relatively little genetic diversity, and may be largely derived from central African chimpanzees (P. t. troglodytes). And central chimpanzees have had substantial gene flow with west African chimpanzees (P. t. verus) but that gene flow has been mainly unidirectional from west to east. This may indicate that the central African population was founded as a result of migration from the west, or that the less known subspecies P. t. vellerosus recently originated in the west and spread western alleles into P. t. troglodytes. Or it may indicate that central Africa on balance is a less successful long-term habitat for chimpanzees, as a result of climate change. In any event, there seems to have been substantial genetic turnover of chimpanzees originating from west to east during the Middle and Late Pleistocene.

Suppose that originally the ancestors of today's chimpanzees were a West African lineage. What could limit them from spreading eastward? There are several possibilities:

  1. Competition from hominids.
  2. Competition from gorillas.
  3. A substantially different adaptation from today, with less colonizing ability.
  4. The partial or complete absence of a central African forest habitat.

Of these possibilities, the first is unlikely to have occurred alone, since hominids probably never had an effective adaptation to rain forests that lacked a substantial understory component. And the last is unlikely because although the central African forest has fluctuated more in extent than the west African rain forest, the Congo basin is very ancient and probably always contained a substantial forested extent.

Competition with gorillas is suggestive, because if ancient chimpanzees were more folivorous (as might be suggested by their large gut) or if ancient gorillas were more frugivorous (with less competition from chimpanzees), the species would have been strong competitors. Unlike chimpanzees, gorillas do not range into hominid habitat. The diets of lowland gorillas are not as well known as those of the mountain gorillas. The latter are known to be mainly folivorous, specializing on vegetarian matter at ground level. But this kind of food is less available outside the mountain rain forest, and is relatively poor in energy content. Thus, even today gorillas and chimpanzees may be more effective competitors than usually assumed. This is confirmed by the rarity with which the two species are found in proximity to each other.

The third alternative is also interesting. One possibility is that the evolution of modern chimpanzees in the early Pleistocene involved an increase in body size which created a more effective terrestrial adaptation. This would imply that ancient chimpanzees were small (perhaps baboon-sized) apes that were primarily arboreal. These apes might have had a substantial geographic distribution before the evolution of modern chimpanzees in West Africa, or alternatively they may have been endemic to West African forests alone in competition with cercopithecoid primates elsewhere.

So we can open some additional alternatives to ponder.

  1. Perhaps ancient gorillas evolved large body size as a strategy to compete with early hominids. As early hominids succeeded in woodland boundary habitat, gorillas faced the choice of maintaining a small size and a primarily arboreal adaptation, or instead growing larger and competing for terrestrial resources. Gorillas chose the latter; possibly chimpanzees chose the former.
  2. Unlike other hominoids, early hominids appear to have had a high rate of predation. This presumably is a result of their relatively poor arboreal abilities, as well as a tendency to range farther from cover. Another factor may have been a relatively more solitary foraging style--perhaps their foods occurred in patch sizes that precluded group foraging. One consequence is a high mortality among young adults in early hominids compared to chimpanzees.
  3. The other major difference is the high fertility of early hominids. Chimpanzees have a birth interval of 5 years or longer; humans with larger body sizes have birth intervals of only 3 years or so. The birth interval of early hominids is not known but we can presume it to have been high to compensate for their high mortality from predation. If chimpanzees faced the same predation, their low fertility would make them poor competitors with early hominids. One possibility is that chimpanzees maintained a higher fertility than today by maintaining a relatively small body size. A Proconsul-sized chimpanzee might have a hominid-like birth interval, a highly effective arboreal adaptation, and a minimal competition with either hominids or gorillas.
  4. It may be no coincidence that the spread of modern chimpanzees directly followed the extinction of the robust australopithecines. Robust australopithecines themselves do not look much like chimpanzees, but many of their differences may be relatively superficial. The dental specializations of robust australopithecines may have derived from a strategy toward fallback foods like seeds in periods of resource stress, in which case their average diet may have been much more similar to chimpanzees than their appearance would indicate. Like chimpanzees, robust australopithecines may have been opportunistic hunters, they may have relied on termites, and they were habitat-limited by competition with early humans. And the body size and energy budget of robust australopithecines was very close to that of living chimpanzees. The advantage of chimpanzees is a more effective arboreality which allows better exploitation of fruits all year and a better resistance to predation. The disappearance of these robust australopithecines therefore left vacant a niche that may have been partly occupied by chimpanzees. This possibility would suggest that robust australopithecines themselves may have had a substantially greater geographic extent than currently known, extending across the Sahel and into the Congo basin in non-forested areas.

All in all, I think it fairly likely that chimpanzees are a very poor model for the human-chimpanzee common ancestor. On the other hand, today's chimpanzees might be a very good evolutionary model for the australopithecines. The major differences between them appear to benefit the chimpanzees, as they have lower predation. Their smaller brains may nonetheless support an equivalent behavioral sophistication to early hominids, since a longer birth interval gives chimpanzees somewhat more time to learn cultural behaviors. And the timing of their spread makes it appear likely that they in fact occupy an ecology more similar to the australopithecines than any other living primate.