Human evolution happened quite rapidly in comparison to other animals such as fish or dinosaurs and is often very difficult to grasp. Many people say: “If we evolved from monkeys, why are they still here?” First off, we did not evolve from monkeys; we simply share a common ancestor with primates, which I will explain here.
To begin with, all humans are hominoids- ‘Great Apes’. Also included in this group are orangutans, chimpanzees, gorillas and bonobos. Genetic studies show that this group, and all general primates, diverged from other mammals in the Late Cretaceous, approximately 65 million years ago. The earliest fossils, however, are from the Palaeocene- 55 million years ago. After this the group split further; the family Hominidae diverged from Hylobatidae (the gibbons) somewhere in the region of 15 to 20 million years ago. As you can see, the millions of years we are looking at here are not a lot in geological terms. A further split was made 14 million years ago when orangutans (Ponginoe) diverged from Hominidae.
Being able to walk upright is a main adaptation that all hominoids have. Bipedalism was first seen either in Sahelanthropus or Orrorin, and these may be our last common ancestor with gorillas and chimpanzees, which diverged around 4-6 million years ago. Bipedals eventually evolved into the australopithecines and the later the genus Homo.
The earliest Homo members are Homo habilis, which evolved 2.3 million years ago. This is the earliest species for which there is evidence of stone tools. Their brains were the same size as chimpanzees’. Over the next million years, there was a period of encephalization; where the brain mass exceeds that related to the animal’s total body mass. This has been directly linked to the increased level of an animal’s intelligence and by the time Homo erectus arrived in the fossil record, cranial capacity had doubled to 850 cm3. This species and Homo ergaster were the first to leave Africa, and spread through Africa, Asia and Europe 1.3 to 1.8 million years ago. These also may have been the first to use complex tools and fire.
After this, modern humans may have evolved from Homo heidelbergensis, Homo rhodesiensis or Homo antecessor (this is according to the Recent African Ancestry theory) in Africa and migrated out of the continent about 500000 to 100000 years ago, which is very recent indeed. These replaced populations of Homo erectus, Homo denisova, Homo floresiensis and Homo neanderthalensis, wiping them out.
Then, Homo sapiens evolved between 400000 and 2500000 years ago. However, it may be possible that Neanderthals and other hominids, may have contributed up to 6% of their genome to present-day humans. This is based on DNA evidence. Modern humans evolved from archaic Homo sapiens 200000 years ago, in the Middle Palaeolithic, after which there was a gradual change in language, culture and technology.
To build on this, the degree of sexual dimorphism reduced over time. This is seen mainly in the reduction of the male canine tooth (except gibbons), reduced brow ridges and the reduction of overall robustness of the males. The evolution of a hidden estrus was also a major physiological change. This means that you cannot tell when the female is in heat and humans and bonobos are the only apes which have this (also where the female is fertile all year round). But, there is obviously still a large amount of sexual dimorphism in humans, such as distribution of body hair and fat, not to mention the fact that males are 15% larger than females.
Other important adaptations that separate humans from other hominoids are that our vision is better than our smell; we have a smaller gut; we have lost a lot of our body hair; we have sweat glands; we have a parabolic dental structure and we have a chin!
Now I have explained how we humans separated from the other apes but you may be thinking: what happened before that? So here is the early evolution of primates…
This can be tracked back about 65 million years ago. The oldest known primate-like mammal was widespread; in Eurasia, North America and Africa in the Palaeocene and Eocene and it was called Plesiadapis. Early primates flourished in Eurasia and migrated south to Africa. The ones that did not migrate and still lived in the tropical regions evolved into lemurs, lorises and bush babies and the anthropoids (the simians).
About 22 million years ago, in the Miocene the catarrhines (a sub-order of higher primates) diversified, and continued to up to 13 million years ago. This explains why there are many other types of monkeys around today and dismisses the: “If we evolved from monkeys, why are they still here?” comment.
The last common ancestor of gorillas, chimpanzees and humans may be fossils found in Kenya of Nakalipithecus and Ouranopithecus from Greece. At about 8 to 4 million years ago gorillas and then the chimpanzees split from the line leading to humans. Human DNA is 98.4% identical to that of chimpanzees.
To conclude, the evolution of man began with the split and migration of early primates. Africa seemed like the prime location for further evolution, whereas those who stayed in Eurasia became lemurs and more primitive apes. There were may species of the Homo family that evolved in Africa but eventually Homo sapiens were the ones that dominates, pushing out all other species, even the ones that had left Africa.
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