Posted on Categories Discover Magazine
Around 6 to 8 million years ago, deep in the rainforests of Africa, humans shared a distant ancestor with chimpanzees and bonobos. Since then, a lot has happened. We climbed down from the trees, stood upright, learned to hunt, found fire and spread across the globe. But how? How does human evolution actually work?
Toward the end of the Miocene — a geological epoch that occurred from 23 to 5.3 million years ago — humans began to diverge as their own distinctive primate from a common ancestor shared with other primates. We know it happened in East and Central Africa in Ethiopia, Chad and Kenya and we think that our first known shared ancestor was Sahelanthropus tchadensis. It’s also been suggested that this may have been the first species to walk upright.
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Like plate tectonics, human evolution allows for big changes, but it takes a while. It happens when slight changes in our genetic material occur over a long period of time. Changes can also happen in the proportion of genes that make up our DNA or mutations in the genes that can occur due to human evolution. Gene expression, or how genetic changes impact behavior and physical makeup, can also cause a species to evolve.
The changes in our DNA are tiny from one generation to the next, but they add up to big changes over time. A sporadic mutation, for example, can be passed on from one generation to the next until it spreads across a family tree. One of the ways that human evolution occurs is by natural selection when certain traits that contribute to successful survival are passed down to the next generation. These changes may first occur as mutations, but the more they happen within a gene pool, the more commonly inherited the trait becomes.
Human height is a great example. Homo habilis is an early human species that lived around 2.3 million years ago. It was much shorter, an average of 4 feet tall. Its height meant it was too small to survive without living in the trees, which is evident from its thick fingers and long arms used for climbing. Over time, and with access to bone marrow scavenged from animals, its brain grew, and so did the species.
Homo erectus, which lived starting 1.8 million years ago, was around 4.8 feet tall, the first species to hunt and the first to live on the ground. It lost its fur to survive the heat of the African savanna and then it spread all over the globe. Homo erectus also used fire more commonly, which made it easier to digest meat and allowed more energy for brain growth. It took nearly a million years for changes in the chemical molecules that we call genes to cause significant changes in both species’ DNA.
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After nearly a million years, Homo erectus vanished and made room for modern humans or Homo sapiens. They were around the same height as us with similar intelligence. They hunted with the composite spear and in groups that allowed them to go after the apex predators that would have hunted them in the early days.
Human evolution goes back long before humans were even humans. It started billions of years ago when the Earth was evolving. We were built on the DNA of the species that came before us, and then mutations and changes grew on top of that. We’re still evolving today to remain alive on our changing planet, which we’ll continue to do as long as humans survive and thrive on Earth.
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