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A Closer Look At: Hominins [1]
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Date: 2025-04-08
The first diary in a series which looks at significant species in the evolution of humans.
This is a diary series in which we take a closer look at hominins, fossil ancestors of the human family.
Humans evolved from apelike ancestors.
In previous centuries, this simple statement may have gotten you burned at the stake for heresy. (And, given the way our political future looks, it may do so yet again.)
This idea was first put forth in 1859, when Charles Darwin wrote On the Origin of Species and spelled out his idea of evolution through natural selection. At that time, most people accepted the religious doctrine of the Christian churches that the first humans, Adam and Eve, had been created from clay by God during the six days of Creation that were described in the Bible, and it was further accepted by most that this had happened somewhere between 6 and 10 thousand years ago. (This was calculated by working out all of the “begats” that had been listed in the Biblical stories.)
By Darwin’s time, however, this dogma had already begun to crack: geologists knew that the Earth was far older than the Genesis account allowed for, and biologists were already speculating that species could change over time, though there were no good ideas yet about how this took place. Nevertheless, Christian churches still held considerable sway in Europe and the US, and Darwin knew he was walking on a minefield. So he avoided the topic completely. In his 500-page tome, he made only a brief and passing mention of humans: “light will be thrown on the origin of man and his history.” But even that was enough. Darwin and his ideas were roundly condemned by conservative fundamentalists, and still are today.
Today, however, we know a lot more about the evolutionary past of Homo sapiens than we did in 1859. Where once there were only a handful of hominin fossils, today there are entire museum displays. And thanks to modern genetic analysis, we have a source of information that can work independently alongside the fossil record. By directly comparing the DNA sequences of living animals, we can determine how closely related they are and how long it has been since they shared a common ancestor.
So this diary series will examine each of the known hominins and their place in human history.
Some definitions first. All living organisms are classified into a hierarchical system that was first laid out in the 1700s by a Swedish biologist named Karl Linnaeus. Under this system, humans are primates, a group of animals which also includes tarsiers, lemurs, gibbons, apes and monkeys. Perhaps surprisingly, there is no good definition for “primate” that works in every case, but in general primates are mammals that have forward-facing eyes giving binocular vision, grasping hands with nails on the fingers and toes rather than claws, and large brains for their body size. Most primates live in extended social groups.
Primates
Within this “Order”, we fall into the Super-Family known as hominoids, which contains the gibbons and siamangs along with the Great Apes, and within these hominoids, we belong to the Family called hominids, which includes only humans and the Great Apes. Today there are only four living Great Apes—the orangutan, the gorilla, the chimpanzee, and the bonobo, which are limited to particular areas of tropical rainforest in Africa and southeast Asia. In ancient times apes were much more diverse and widespread.
Today the modern world is dominated by mammals, but that was not always true. For over 150 million years, from the middle of the Triassic to the end of the Cretaceous, this was a dinosaur planet. The dinosaurs had representatives in nearly every terrestrial niche, and their close relatives were the dominant life in the sea and in the air. The mammals which existed during that time were mostly small and rare, and survived by being unobtrusive and active at night. It wasn’t until the extinction of the dinosaurs at the end of the Cretaceous, 66 million years ago, that the mammals were finally able to radiate and adapt to fill all of the now-empty ecological niches. And one of these mammal groups was the primates.
Though genetic analysis indicates that the earliest primates probably lived in Eurasia as long as 80 million years ago, the earliest known fossil of a proto-primate, Purgatorius, was found in what is now Montana. It was a small animal similar to modern tree shrews, that lived around 65 million years ago, just after the asteroid strike that had killed off two-thirds of all life, including the dinosaurs. Unlike most of the early mammals, who were insectivorous, Purgatorius teeth were adapted for eating fruits. Its front paws had grasping abilities which allowed it to grip tree branches and the bones in its ankles allowed the joints to swivel, enabling the animal to climb trees easily and effectively and take advantage of this food source that had become more widely available as flowering plants adapted themselves to the new post-Cretaceous world. However, Purgatorius did not yet have forward-facing eyes like today’s primates.
Sometime around 55 million years ago, the primates had become better-adapted to a life in the trees, and fossils of lemur-like animals with grasping hands, long tails, and forward-facing eyes have been found in Eurasia. One of these is Darwinius, nicknamed “Ida”, which lived in Europe about 47 million years ago. It is known from only one fossil, found in the German Messel deposits, but it is spectacularly preserved, with a full coat of fur and even its internal organs plainly visible. Because Ida lacks the cleaning claw on its toe and the grooming comb found in lemur teeth, it is probably not in the direct line to lemurs, but may be an ancestor of the tarsiers, and its ankle bones have some features that are found in later monkeys.
The “Ida” fossil, on display in Oslo
These early lemur-like primates managed to spread across the Indian Ocean to Madagascar. They survived there to the present because the deep water and currents prevented any ancestral monkeys from rafting to the island and establishing themselves there (which is why modern Madagascar has lemurs but no monkeys or apes).
A short time later, the Eosimians appeared in Asia, and these are held by most taxonomists to be the early ancestors of the monkeys. They made their way across Asia to Africa and Europe, where the Parapithecids appeared about 34 million years ago. These were replaced by the Propliopithecids, and their descendants are classed as the Old World monkeys.
One of the earliest of these Old World monkeys was Aegyptopithecus. Found in Egypt and dated to 30 million years ago, Aegyptopithecus was about the size of a modern female Howler Monkey and weighed around 15 pounds. It was adapted for living in trees and was probably a fruit-eater, and it is pretty representative of the monkeys that lived at the time.
3d print of Aegyptopithecus skull. Note the large canine teeth.
By 25 million years ago, a group of Old World monkeys had reached South America, either by crossing a series of islands or by rafting across on vegetation mats (at this time, the Atlantic Ocean was some 600 miles narrower than it is now and had several strings of islands extending from its shores). Here they evolved into the New World monkeys, which differ most significantly from African monkeys in having prehensile tails to aid them in climbing.
A significant fossil in the evolutionary history of primates was Proconsul, first found in Kenya in 1909 but not recognized until the 1940s. It dates to about 22 million years ago. Proconsul lacked a tail and had a brain volume that was much higher compared to its body size than monkeys—the hallmarks of an ape. (The name itself reflects this: at the time the fossil was scientifically described there was a popular Chimpanzee on exhibit at the London Zoo named “Consul”. And so, as the proposed ancestor of the apes, Proconsul was “before Consul”.) In addition, the molar teeth of Proconsul have the “Y-5” cusp pattern that is typical of both African apes and hominins, where the molar teeth on the lower jaw have three cusps at one side and two on the other, separated by a Y-shaped groove.
Proconsul. Note that the canine tooth is slightly smaller than in Aegyptopithecus.
Unlike monkeys, which usually move by running across the tops of branches or across the ground on all fours, using their tails for balance, apes have their own specialized ways of moving. Proconsul’s arm and shoulder joints were arranged like those of a modern ape to allow them to hang straight down by their arms, and this allows them to move through trees by “brachiating”, or swinging hand over hand from branch to branch. (On the ground, the Great Apes have another method of locomotion called “knuckle walking” in which they put their weight on their folded fingers, but this is a specialization which appears to have evolved after the ape/human split and is not found in the early apes.)
Many authorities have concluded that Proconsul (or Ekembo, the new genus that was later assigned to several of the fossils) is ancestral to the African apes and, indirectly, 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 (which also have a Y-5 molar pattern), that migrated to Africa from Europe during the climate changes of the late Miocene era.
Ramapithecus/Sivapithecus. Note the relatively small canine tooth.
Another significant fossil ape was found in 1932 in the Siwalik Hills in what was then India and is now Pakistan. It was given the name Ramapithecus, and it dated to around 12 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, and the reassembled lower jaw looked like it had the rounded-V-shape parabola form found in humans rather than the U-shape seen in apes—leading researcher Elwyn Simons to conclude in 1960 that Ramapithecus was a very early member of the hominin line. In the 1970s, however, as more and more fossils were found, it was discovered that all of the fossils which had been attributed to Ramapithecus were actually females of a different species, Sivapithecus, and the reduced canines were simply a sexual characteristic that is found in most modern apes (the males have long canines to use as social threat displays, which the females do not do). It did not indicate any hominin affiliations. A short time later the first genetic studies concluded that the split between apes and humans had happened much more recently than Ramapithecus, perhaps only 5 million years ago (an estimate which has since been revised upwards). Based on these findings, Ramapithecus was folded into Sivapithecus, and today most taxonomists conclude that Sivapithecus is an early Orangutan ancestor.
According to the evidence from genetic analysis and from fossils, one of the hominid lines branched off from the African apes between 7 and 8 million years ago into two separate lineages. One of these went on to produce the modern Chimps and Bonobos (and they remained hominids). The other went in a different direction and eventually produced humans. That branch is the “hominins”, consisting of all members of the human branch after the split from the apes. This includes our own genus, Homo, as well as several other extinct genera.
Hominins
In future diaries, we will take a closer look at individual hominin species.
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