Sunday 4 July 2021

Miocene (Pt 27): Rise of the Apes

From a human perspective, one of the most significant evolutionary developments of the Miocene epoch was the appearance of the first apes. Exactly when the group first arose isn't entirely clear, but it's either very early in the Miocene or very late in the preceding, Oligocene epoch. Part of the reason for the lack of clarity is, as so often, dispute as to where exactly the dividing line is between apes and monkeys when we go this far back in time. But there also seems to be some evidence that the earliest apes evolved in African jungle habitats, which weren't the best for forming fossils.

An example of this early confusion comes from Dendropithecus, from the Early Miocene of Kenya. Comparatively small, at only around 60 cm (2 feet) in length, and with arms that seem to be adapted to swinging from trees, when it was first named as a distinct genus in 1977, it was thought to be an ancestor of modern gibbons. Despite having a somewhat similar lifestyle and diet, this no longer thought to be likely, and one recent analysis places it as belonging to a very early branch in the ape family tree - just early enough that one could legitimately argue as to whether it really counts as an 'ape' or just a very close relative.

The group to which it belonged survived through almost the entire Miocene, although it left no modern descendants. It includes the Middle Miocene Simiolus minutus, with an estimated body weight of 3.5 kg (8 lbs), about the same as a domestic cat. This is currently thought to be the smallest known species of ape, beating out the previous record-holder, Micropithecus, whose status as an ape, rather than an odd monkey, is even less clear.

Perhaps the best-known of the Miocene apes is Proconsul, first formally described in 1933. For much of the 20th century, this was thought to be an ancestor of both the gibbons and the great apes, but it's now thought to represent an early branch dividing off before they split from each other, and not a direct ancestor. A considerable number of other ape genera have been placed in the same family, or even the same genus, as Proconsul at one time or another and the exact status of some of them remains in flux. 

One of these possible relatives is Kamoyapithecus, which actually lived at the end of the Oligocene and, if it really is an ape, is probably the oldest one known. Another is Nyanzapithecus, although a 2020 study on the skull of an infant showed that this might actually be more closely related to modern apes. It had a gibbon-like face and teeth but, from what little we know of its limbs, seems to have moved much more like a great ape, hinting that it might be close to the ancestry of both groups.

If we could see it in life, there is little doubt that we would consider Proconsul to be an "ape", even if it didn't belong to either of the families that survive today. Compared with typical monkeys, it had a large brain and, perhaps most significantly, it did not have a tail. There's also the matter of the large size. While at least some species seem to have been sexually dimorphic, complicating the picture, most ranged from about 20 to 35 kg (45 to 80 lbs), putting them on a par with baboons, the largest living monkeys. Indeed, one species, P. major, has been estimated to have weighed around 75 kg (165 lbs) - about the same as a female gorilla.

On the other hand, the shape of the limb bones suggests that Proconsul and its close relatives walked in the way that macaques and baboons do, rather than the distinctive style of living apes. It also probably spent more time in the trees than great apes (or baboons) do, helping to feed its heavily fruit-based diet.

Regardless of which species we count, there seems little doubt that apes originated in Africa. However, when that continent collided with Eurasia in the Middle Miocene, a number of species headed into the new lands. These presumably included the early ancestors of the gibbons, which molecular evidence suggests diverged from the great apes somewhere between 20 and 18 million years ago. However, we know almost nothing about what these creatures were like, with most fossil gibbons being much more recent. Essentially all we have from the Miocene is a single 13 million-year-old gibbon tooth recovered from India and described just last September. 

Gibbons were not, however, the only apes in Miocene Eurasia. Dryopithecus is known to have lived from at least Spain to Hungary around 12 million years ago, and a number of other European apes are known from around the same time - many of them probably related. Once again, there is debate as to where it fits in the ape family tree, although it is almost certainly a great ape of some kind. It, and a number of close relatives such as Danuvius from Germany had a gorilla-like face, but limbs that more closely resemble those of orangutans. For that matter, there are indications that it was at least capable of walking on its hind legs for brief periods, although its hard to know how often, or how effectively, it did so.

It may well be that these European great apes belonged to a third branch that is no longer around, dying out as the European climate changed in the Pliocene, although arguments for relationships with with the living African and Asian branches have certainly been made. Dryopithecus itself was about the size of a chimpanzee, and most of its relatives were broadly similar. Its teeth suggest a generalist diet, it being able to eat a wide range of food types, but its favourite seems to have been fruit, and one Austrian fossil ate so much sugary fruit when it was alive that it developed dental caries.

Hispanopithecus, which lived in Spain around 10 million years ago, is another likely member of the group, although somewhat larger than its relatives, and with some adaptations that parallel those in unrelated later apes, giving it a style of movement somewhere between the two, using both ape-like climbing and monkey-like walking. The areas it lived in were positively swampy and subtropical, with plenty of reeds, palms, and fig trees; it seems to have mostly eaten soft food such as fruit. Similar in size to a chimp, the males are thought to have weighed around 45 kg (100 lbs), but the females were only around two-thirds of that, and with smaller canine teeth.

Further east lived the ancestors of modern orangutans. The oldest of these is Sivapithecus from Pakistan and northwest India, which, with the oldest specimens aged a little over 12 million years, comes very close to molecular estimates of the origin of the orangutan clade. It was similar in size to modern orangutans, although its arms were not quite so well adapted for hanging. The hind feet likely had a powerful grip, using the big toe like a thumb, but there is some evidence that may have knuckle-walked as gorillas and chimps do - and living orangutans don't.

Sivapithecus went extinct around 9 million years ago, as the dense forests of the region faded to give way to the more arid habitat that it retains to this day. It was once thought (under the now-defunct name Ramapithecus) to be a possible ancestor of humans and, while it clearly isn't that, quite how closely related it really is to orangutans is still not entirely settled, with some arguing that it might be the result of an ultimately doomed migration of African apes into Asia. Possible later relatives include Ankarapithecus from Turkey, Khoratpithecus from Thailand, and Lufengpithecus from China, all of which lived in the Late Miocene. 

The last wild ape in Europe was likely Oreopithecus, which lived in southern Italy from 8 to 6.5 million years ago towards the end of the Late Miocene. The size of a small chimpanzee, it lived in relatively open woodland, implying a diet rather different from most modern apes. Much has been made of whether or not it was unusually human-like, and what that might imply about its position in the ape family tree.

Early studies of the known fossils showed that it was capable of at least some degree of upright walking, and moreover that its opposable thumbs allowed a hand grip of a decidedly human form. This view has by no means been entirely overthrown, but it seems to be in a minority these days; the skeleton of the lower  back does not seem to be suited for bipedal motion, the shape of the feet may be ill-suited for walking, and the grip may have had the strength of an ape, but not the precision of a human. The most common modern opinion is therefore that, while it's undoubtedly a great ape, it's more likely an evolutionary side-branch than the ancestor of anything alive today.

The ancestors of the African apes must, however, have been living on that continent at the time. We know of surprisingly few of them, probably because they didn't live in areas conducive to fossil formation, but one exception is Chororapithecus. Living 9 million years ago in Ethopia, this is only known from a partial set of teeth, so it's hard to be sure of what exactly it was. But the teeth are almost identical in size to those of a gorilla, and have a number of similarities in their shape, so it's at least possible that is, in fact, the only known Miocene 'gorilla', although its age would put it almost exactly on the estimated time that gorillas split from the group that includes chimps.

To find members of that latter clade we have to head very close to the end of the Miocene. One suggested candidate is Graecopithecus which lived, not in Africa, but Greece, around 7 million years ago. Known only from a jaw, it has been proposed that this animal may have been a human ancestor, its immediate descendants presumably having headed back to Africa during the Pliocene epoch. (It's also possible that this is the same ape species that left the fossilised footprints that were in the news a few years back, but these were 1.5 million years younger than the jaw, so it's hard to say for sure). It's fair to say that this proposal is controversial and that we'd need rather more remains to clearly judge it one way or the other.

Whether it has anything to do with Graecopithecus or not, it certainly is around this time that chimps split away from our own ancestors. Apes such as Sahelanthropus and Orrorin lived in the last couple of million years of the Miocene and seem to have slightly more in common with Pliocene Australopithecus than with chimpanzees, with the latter in particular having some evidence for fully bipedal walking. But the remains are too limited to be really sure, and there are other possibilities. In any event, that question is beyond the scope of this blog, and very nearly beyond the end of the Miocene.

Which, as possible proto-humans march on into the Pliocene, brings me to the end of another round of Miocene coverage. But there are still more continents to cover if we are to see the full scope of the world in that time, so it's time to head back over the Atlantic to see what was happening in South America...

[Photo by Guérin Nicolas, from Wikimedia Commons.]

1 comment:

  1. A note re: Miocene South America is that the Late Miocene marks the start of a long-term decline in South American predator lineages, which would culminate in the Early Pliocene with the entire large-bodied predator guild all but completely collapsing (note that this is prior to GABI)