Since Darwin revealed his seminal theory of evolution by natural selection, human beings have endeavored to understand their own evolutionary origins and history. A lot of questions still remain, but these mainly pertain to the specifics. Today, paleoanthropologists understand in great detail the evolutionary emergence of a number of traits that we consider, at least superficially, unique to modern humans.
Human beings and chimpanzees, our closest living genetic relatives, are thought to share a common ancestor that lived 6 to 8 million years ago. This is based on genetic differences, where anthropologists have compared rates of mutation in both species and estimated how long this might have taken to result in the genetic differences we see today, resembling something of a genetic clock.
Since then, multiple branches in the tree of human evolution have come and gone, while some had descendants who evolved into species that eventually became modern humans.
What Is Human Evolution?
Human evolution refers to the long and complex process by which modern humans, Homo sapiens, have developed from earlier, more primitive species of hominins (the group that includes modern humans and their ancestors).
The term "hominins" encompasses a diverse array of species, including not only our direct ancestors but also those with whom we share a common lineage. These hominins walked the Earth long before the emergence of modern humans and exhibit a fascinating mosaic of traits that connect our roots.
First Bipedal Hominin: Ardipithecus ramidus
(7 Million to 4 Million Years Ago)
Ardipithecus ramidus is one of the earliest hominin species to exhibit clear evidence of bipedalism — walking on two legs. One of the most obvious adaptations that separates humans from our close genetic relatives is bipedalism. While other primates occasionally walk on two legs, we are the only primates who do it regularly as our preferred way of moving.
"The first hominins we see appearing 7 million to 4 million years ago, they seem to be walking on two legs, but they are probably short, like 4 feet tall," says Bridget Alex, a paleoanthropologist from Harvard University's Department of Human Evolutionary Biology. (Alex is also a regular Discover contributor.) Most fossil remains from the period are fragmentary, with parts of the skull or tooth providing anthropologists with evidence of their physiology.
Read More: What Did Ancient Humans Look Like?
First Culture and Hunting Tools: Australopithecus
(4 Million Years Ago)
Alex says a number of early human fossils from a group of species called the Australopiths that date back 4 million to 2 million years were living in what are now dry regions in Africa. By this time, early human species were showing the first signs of material culture, with stones being fashioned into rudimentary knives and scrapers.
A common hypothesis is that this was a savannah-like environment where our ancestors could hunt during the heat of the day, avoiding other predators who might come out in the early morning and evening when it was a bit cooler. Being bipedal also meant less of our body was exposed to the Sun and was more adept at covering longer distances with less energy.
Another conspicuous difference between humans and other primates is that we look naked. We have lost our thick dark fur, and researchers have established that this is linked with sweating.
"We are the sweatiest primate by far. We have way more sweat glands than any other mammal. This, combined with our light hair, allows the sweat to evaporate easily, which has a cooling effect," says Alex.
First Controlled Use of Fire: Homo erectus
(2 Million Years Ago)
Homo erectus is known for its controlled use of fire, advanced toolmaking skills and the ability to migrate out of Africa. We have evidence of the first sporadic use of fire, with discolored sediments found in Koobi Fora, Kenya. An often overlooked factor that likely greatly influenced human evolution is that our ancestors decided to start cooking their food.
Unless you can afford to buy the oils and supplements necessary, modern humans need to eat cooked food.
"Without cooked food, women will lose their periods and become unable to reproduce in a natural environment, so the human body is adapted and dependent on cooked food," says Alex.
Cooking our food breaks it down mechanically and chemically, making it easier for our digestive system to extract energy from it. This likely helped feed our growing, metabolically expensive brains with a steady energy source. Even other animals will often prefer to eat cooked food over raw food if given the option.
"And then when you get to 2 million years ago, you see a species called Homo erectus, and at this point, they are probably as tall as living humans. They have much smaller brains, but from the neck down, they look very much like modern humans," she says.
During this time, we see gradual innovations in material culture where tools and technology are getting increasingly sophisticated. H. erectus is also thought to have migrated out of Africa in large numbers.
Early anthropologists thought our large brains must have been the first change, but it actually comes kind of late in our evolutionary story. There is a gradual increase in brain size between the Australopiths and H. erectus, but after that, we see an increasing brain size until about 100,000 years ago when cranial capacity basically maxes out.
Modern Human Interbreeding: Homo Neanderthalensis
(250,000 Years Ago)
Evidence reveals a common genetic heritage between Homo Neanderthalensis and modern humans, which indicates they interbred.
"Genetic evidence suggests Neanderthals and other Homo species had low genetic diversity, similar to endangered species today. Whereas Homo sapiens had greater genetic diversity, a larger population, and occupied different parts of the world by the time they were interacting with these other species, with the others being more vulnerable to competition from another human species," says Alex.
If we were to go back 100,000 years, then there would be at least four different species of humans living on the planet; Homo Neanderthalensis, Homo sapiens, Homo Floresiensis and the Denisovans. But by 40,000 years ago, there was only one species left, and that's us, Homo sapiens.
There have been a number of hypotheses put forward by scientists to explain why we became the last remaining species of humans on the planet; however, the answer is probably a complex mix of different factors.
Neanderthals and the Denisovans lived in ice-age Europe and Asia, respectively, meaning they likely had small populations due to living in extreme environments, whereas H. sapiens were more spread out, with a large population living in modern-day Africa. This, together with competition from H. sapiens, who likely possessed technologies like projectile weapons, likely contributed to these other human species going extinct. We do also know, however, that interbreeding did happen, and we see evidence of this in modern human DNA.
We also can't really talk about human evolution without mentioning the development of language. Cognitively, the starkest difference between humans and primates is language, at least in the way humans use it.
Other species can obviously communicate, but H. sapiens appear to have a unique capacity to acquire language in the form of applying meaning to abstract sounds and intuitively understanding hierarchical linguistic structures. It is an innovation that has allowed us to become the species we are today. It has enabled the development of culture and knowledge, which are perhaps the most important factors that shape the evolution of modern humans.
Read More: How Much Neanderthal DNA Do Humans Have?