Hominins
So, over the past few months I have been downloading some scanned 3d files of hominin skulls (human ancestors) and printing them out. A number of museums have made their scans available to the public, and anybody with a 3d printer can now have their own replica skulls. Although the resolution is not as good as a professional cast, it is enough to illustrate some of the features of each species, and to demonstrate the history of humans from our earliest known ancestor up to today. So how about a quick crash course in human evolution.
[Note: the entire subject of “human evolution” is amazingly contentious, partly because hominin fossils are rare and hard to interpret, and partly because the field involves a lot of researchers with healthy egos who all have their pet theories (usually based on finds they made themselves) and who compete fiercely for publicity, funding, and research permits. So virtually everything here is disputed or challenged by somebody or other, and although the general line of descent is clear (from an apelike ancestor through Australopithecus to early Homo to modern humans), there is much debate and argument over what species is what and who is related to who. All I can do is present the majority view as it now stands. It may all change tomorrow with new finds. That is how “science” works.]
Aegyptopithecus, Proconsul, Sivapithecus
One thing every researcher can agree on is that modern humans, Homo sapiens, evolved from monkey-like ancient primates. Specifically, humans evolved from apes—primates which are tail-less, live in social groups, and have relatively large brains for their body size.
Aegyptopithecus lived in Egypt about 30 million years ago. A tree-dweller about the size of today’s Howler Monkeys, it was typical for the ancient monkeys that lived just before the ancestors of apes split off. Some taxonomists argue in favor of lumping Aegyptopithecus into the Propliopithecus genus.
Proconsul appeared in eastern Africa around 21 million years ago. There are four recognized species, though some taxonomists have argued in favor of splitting two of these into their own genus, Ekembo. Proconsul has several anatomical traits that are similar to the apes, and many authorities have concluded that Proconsul (or Ekembo) is ancestral to the African apes and, ultimately, to humans. Other researchers, however, place Proconsul as a side branch, and conclude that the African apes are descended from a Eurasian group, perhaps the Dryopithecines, that migrated to Africa from Europe during the climate changes of the late Miocene era.
In 1932, a fossil skull was found in the Siwalik Hills in what was then India and is now Pakistan. It was given the name Ramapithecus, and it dated to around 15 million years ago. The find was of particular interest because the canine teeth on the skull, which are long and sharp in apes, were much smaller and rounded, as they are in humans--leading most researchers to conclude that Ramapithecus was a very early member of the hominin line. In the 1970s, however, as new fossils were found, it was discovered that all of the attributed Ramapithecus fossils were actually females of a different species, Sivapithecus, and the reduced canines were a sexual characteristic and did not indicate hominin affiliations. Today most taxonomists conclude that Sivapithecus is an early Orangutan ancestor.
Sahelanthropus, Ardipithecus ramidus, Kenyanthropus
Comparison of DNA sequences has demonstrated that human ancestors split off from the ancestors of modern Chimpanzees about 7 million years ago. It has only been in recent years, however, that fossils have been found from near this time period.
Sahelanthropus was found in Chad in 2001, and dates to sometime between 6 and 7 million years ago. It consists of a badly-crushed skull. When reconstructed, though, the skull seems to show a foramen magnum (the hole through which the spinal cord enters the skull) centered in the bottom of the skull as in humans rather than at the back as it is in apes and monkeys. This indicates that Sahelanthropus walked upright as a biped, which places it as one of the earliest human ancestors. This has been disputed, however, by others who argue that the reconstruction was inaccurate and the skull falls onto the “ape” side of the split.
Another slightly younger find, called Orrorin, has also been claimed as “the earliest human ancestor”, and dates back to 5.8 to 6.2 million years ago. This find consists of just a few arm and leg bones, with no skull. Some studies of the bones have concluded that they belonged to a biped, while other studies have concluded they did not and that Orrorin is an ancient ape.
The next-oldest discovery is Ardipithecus. Two species have been assigned to this genus. Ardipithecus kadabba dates to 5.8 million years. No skull has been found, but study of the foot and arm bones indicates that it may have been adapted for bipedal walking. Yes, this is disputed by some. The other contender for “oldest hominin species” is this one, Ardipithecus ramidus, dating to 4.4 million years. The skull has a foramen magnum indicating a bipedal stance, and the accompanying pelvis and foot bones have also been interpreted as bipedal. These finds are all fragmentary and crushed, though, and some authorities have questioned this interpretation.
Finally there is Kenyanthropus, found in Kenya and dating to 3.3 million years ago. This was around the same time as Australopithecus afarensis (the famous “Lucy”), and there is now debate over which of these was more closely related to humans. Kenyanthropus is difficult to interpret because the skull was badly broken during preservation. The unusually flat face is not what we would expect from a human ancestor of the time, but the skull also exhibits small molar teeth and low brow ridges, which seem to be more humanlike than “Lucy”. However, a few other researchers have concluded that Kenyanthropus is itself just a distorted and badly preserved A. afarensis.
Australopithecus afarensis, Australopithecus africanus, Taung Child
“Lucy”, Australopithecus afarensis, is probably the most famous hominin fossil ever. Found in East Africa in 1974, she is one of the most complete ancient skeletons, and exhibits clear adaptations for upright walking, though her brain capacity was chimp-size and the curved bones in her hands and feet indicated that she still spent a lot of time in the trees. She lived between 3 and 4 million years ago.
Australopithecus africanus lived in South Africa about 3.3 million years ago. It is similar to “Lucy”, but is bigger and has a larger brain and smaller teeth. First discovered in 1924, it was initially rejected as a human ancestor by most authorities because the Piltdown Man, found in England, seemed to be a better candidate. Eventually, Piltdown was found to be a hoax, and A. africanus was finally accepted as a member of the hominin lineage.
The first A africanus to be found was the Taung Child, consisting of the skull of a young toddler and, remarkably, a natural cast of the brain produced by solidified sediment. The brain cast shows anatomical similarities to human brains. The skull has two puncture wounds in it which match the size and spacing of extinct Eagle claws, indicating that the Taung Child had been killed by a large bird of prey.
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Paranthropus aethiopicus, Paranthropus boisei, Paranthropus robustus
These three closely-related species date collectively from around 1 to 3 million years ago. The genus name means “next to man”, reflecting the majority view that they are not in the direct line to humans but are a side branch that died out without leaving any descendants. Although their bodies were not any larger than their contemporary hominins, Paranthropus were characterized by their heavy skulls and massive jaw muscles with huge teeth, indicating a diet made up at least partially of tough fibrous plant stems, tubers, and nuts. The bony ridge at the top of the skull is the “sagittal crest”, where the powerful jaw muscles were attached.
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Homo habilis, Homo erectus, Homo ergaster
Dating from 2.4 to 1.4 million years ago, Homo habilis is the earliest known member of our own genus Homo. The name means “handy man”, a reference to the fact that crudely-made stone chopping tools were found in the same geological layers, which it was assumed had been made by H habilis. (The earliest stone tools are over 3 million years old, however, and might have been made by an australopithecine, though it is not clear which ones.) Although considered to be the earliest Homo, habilis still exhibits a number of australopithecine traits, including a small brain size, long arms, and a face that projects forward. Some taxonomists split the early H habilis off into a different species, Homo rudolfensis: others argue that this is just variation within the habilis species.
Homo erectus is one of the most important species in human evolution. Appearing around 1.9 million years ago, it was the first species to exhibit a basically “modern” human body plan, with short arms, long legs, a big brain, a modern bipedal gait, small teeth and jaws, and carefully worked “handaxe” stone tools. Perhaps because of these adaptations, erectus became the first hominin to leave Africa, becoming established in Europe and as far away as China and Indonesia (where it is known as “Peking Man” and “Java Man”). It used to be thought that each of these geographical versions of H erectus evolved on their own into different races of Homo sapiens, but that hypothesis is no longer widely accepted (though the nationalist CCP in China still likes the idea). The last Homo erectus did not die out until around 100,000 years ago. They were a very successful species.
Many taxonomists now split the African populations of Homo erectus off into their own species, Homo ergaster (the name means “working man”), basing this on some anatomical differences between the African and Asian populations. Other authorities, however, have rejected this idea, arguing that the anatomical differences are mere normal variation within Homo erectus.
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Homo georgicus, Homo floresiensis, Homo naledi
During the 1990s, a series of skulls were found near the city of Dmanisi in the Republic of Georgia dating to around 1.8 million years. Most of the skulls were attributed to Homo erectus. But one of the skulls differed from the others, particularly in its noticeably smaller brain size, and it was given the designation Homo georgicus. Others have argued that it is either a variant of H erectus/ergaster or a subspecies.
In 2003, researchers found an odd skeleton on Flores Island in Indonesia, in deposits dating to roughly 100,000 years old. It appeared to be an adult hominin, but it measured only 3.5 feet tall and had a primitive-looking skull. There was speculation that it might be some sort of medical pathology, but additional skeletons put an end to that idea. Because of their small size, they were quickly dubbed “the hobbits”. Stone tools indicate that the little hominins were living there as long as 190,000 years ago, and died out as recently as 50,000 years. Since then, Homo floresiensis has been the topic of controversy. It has been speculated that they are “island dwarfs”, descended from an isolated population of Homo erectus. Some people have even postulated that they survived till modern times, giving rise to the Indonesian legends of Orang Pendek “wild apemen”. This speculation was fueled further when similar dwarf fossils as recent as 50,000 years were found in the Philippines and given the name Homo luzonensis. (Yes, this is disputed too, with some arguing that they are the same species.)
In 2013, another oddity appeared, when excavations at Rising Star Cave in South Africa found a series of skeletons. Dating from 335,000 to 235,000 years ago and measuring just 4 feet tall, Homo naledi became the subject of some extraordinary claims: they had intentionally buried their dead inside the cave, they had carved simple art designs into the walls, and they had used fire. (Though all of these claims have been questioned and challenged). In addition, they had an odd mix of primitive australopithecine traits and more modern human, making it difficult to deduce their place on the family tree.
Homo heidelbergensis, Homo antecessor
Through the 80s and 90s, a number of skulls were found in Africa and Europe dating between 800,000 and 1.3 million years ago which, though modern-appearing, also exhibited what seemed to be some primitive traits inherited from H erectus (or H ergaster). They were usually referred to as “archaic Homo”. It wasn’t until recently that this “wastebasket” was assigned the name Homo heidelbergensis, which had first been applied to a jawbone that was found in Germany in 1908. Many authorities now conclude that heidelbergensis appeared in Africa and spread to Europe, and was probably ancestral to both Homo neanderthalensis in Europe and Homo sapiens in Africa.
Some taxonomists argue that some populations of heidelbergensis should be split into their own species, including H steinheimensis and H cepranensis. Others argue that the early heidelbergensis in Africa should be split off into the species Homo antecessor, and that it was antecessor who went on to speciate into Neandertals and modern humans.
Homo neanderthalensis, Homo sapiens
In 1856, workers at a quarry in Germany found what they thought was a human skull with thick bones and heavy brow ridges. After Darwin published his theory of evolution, the skull was recognized as an extinct primitive human, and was named Homo neanderthalensis. For a time, it was believed that Neanderthals may have been the direct ancestor of modern humans or even a subspecies. Today most taxonomists consider it a separate “sister species” that evolved away from our common ancestor to deal with the cold conditions in Ice Age Europe. Neanderthals died out around 40,000 years ago. But modern Homo sapiens still carry Neanderthal genes—the product of interbreeding.
In 2008, a finger bone was found in Denisova Cave in Russia and was assumed to be Neanderthal. When its DNA was sequenced, however, it was found to be different from both Neanderthals and modern humans, and was an entirely different species. Today this “ghost species”, known only from its DNA, is referred to as “Denisovans”.
So, the sequence from “ape” to “us” seems to be, at least for now, roughly: Sahelanthropus, Ardipithecus ramidus, Australopithecus afarensis, Australopithecus africanus, Homo habilis, Homo ergaster, Homo antecessor, Homo sapiens. But nearly every one of those steps is disputed, and I suspect there are a lot more side branches and radiations that we don’t know about yet, and probably a lot of cross-branch inter-species shenanigans (we know from DNA studies that Denisovans, Neanderthals and modern humans all interbred with each other). What we need are more fossils to fill in the gaps.
In future diaries, I will take a look at individual skulls in more detail.
NOTE: As some of you already know, all of my diaries here are draft chapters for a number of books I am working on. So I welcome any corrections you may have, whether it's typos or places that are unclear or factual errors. I think of y'all as my pre-publication editors and proofreaders. ;)
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