The history
of human evolution is a really complex subject, but I will do my best to answer your question. I
encourage you to have a look at the full scientific classification of humans, beginning with
the kingdom Animalia at the most general, and ending with our species, Homo
sapiens, at the most specific.
Without going too far, back, let's
consider the ways in which Primates (the order we belong to) are different from other mammals.
All mammals are warm-blooded (or endothermic) animals which have hair and produce young for
their milk. In addition to these features, Primates have some skeletal structures which are well
adapted to life spent in the trees. Even our species, which live on the ground, retain some of
these features in our forelimbs and cranial morphology because our ancestors were arboreal. In
fact, our stereoscopic vision developed as a beneficial adaptation to life in the trees.
Stereoscopic vision allows for better depth perception-- in other words, our ancestor species
were better able to see and grasp branches in the trees.
The diversion of
Primates from other mammals occurred about 63 to 74 million years ago. Fast forward several
million years to about 15 to 20 million years ago, with the emergence of the Hominids. The
Hominids, or great apes, are tailless primates, with forward-facing eyes, and downward-facing
nostrils. Our "cousins" in the Hominid family are gorillas, chimpanzees, orangutans,
and bonobos. Around 8 million years ago, the ancestors of modern humans, chimpanzees, and
bonobos diverged from the gorillas. We call this group of chimpanzees, bonobos, and humans the
tribe Hominini.
Here's where "we" start to take shape! Around 7
million years ago, humans and chimpanzees started to evolve in their separate directions. Around
this time, species like Sahelanthropus tchadensis emerged as the earliest
non-Pan (chimpanzee) ancestors of modern humans. Some important changes took place
between then and now. First, hominins started leaving
the trees! Not altogether, but around 4 or 5 million years ago, our ancestors started spending a
lot more time on the ground. We know this from changes in the structure of the skeleton which
enabled semi-upright walking, at least for a little while. Changes in diet were happening, too,
which we know from the structure of the jaw and teeth. In addition to bugs and fruit from the
trees, our ancestors started eating some nuts and grasses.
Around 3 million
years ago, Australopithecines like Australopithecus afarensis began
spending more time walking upright on the ground than in the trees. Not much later, our
ancestors learned how to control fire and create stone tools. Both of these skills allowed them
to begin processing and cooking foods, which offered a higher nutritional reward for their
efforts. There emerged a reflexive relationship between stone tool creation and use, brain size,
and the development of culture-- when we got better at one, we got better at the others! Some
anthropologists believe that it was our new-found ability to process and cook the meat of other
animals which really drove the emergence of our own genus, Homo.
The first of the Homo genus was Homo
habilis-- tool-users who lived from about 2.8 to 1.5 million years ago. Later, about
2 million years ago, Homo erectus emerged and began migrating out of
Africa, into Europe and Asia. H. erectus is one of our most likely direct
ancestors, though there is a lot of variation to take into consideration! Our own
species, Homo sapiens, emerged about 200 thousand years ago and co-existed
for some time with Homo erectus! However, "modern humans," in the
way that we think of ourselves, did not migrate out of Africa until about 60 thousand years
ago.
Since Homo sapiens has been around, we've been
pretty busy. For about 50 thousand years, we've been creating art and musical instruments.
Around 12 thousand years ago, we began practicing agricultural and developing settlements.
Around 5 thousand years ago, we started writing things down, and the rest-- as they say-- is
history!
If you want to learn more about human evolution, check out the
Smithsonian's website on Human Origins. They have a lot of great diagrams, timelines, and
interactive material to cover our very long story!
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