An introduction to us

No matter where we come from, every human alive today is a member of a single species called Homo sapiens. We’re a species of mammals with the ability to use symbolic language and tools, and we’re a species capable of collective learning. We have large brains, stand upright, walk on two legs, and we have opposable thumbs capable of very precise movements. We organize ourselves in societies and create cultures.

Like other life on Earth, our species evolved from ancestors who were different from us. In fact, there have been many different species of hominins. Hominin is the word we use to refer to all the species that are our close genetic relatives. In other words, hominins are all the species of humans—including modern humans such as ourselves as well as our extinct ancestors.

That’s right, we’re not the only humans. Once, Earth was home to many different species of humans. Others include our distant relatives—for example, the australopithecines—and closer relatives, such as Homo erectus, Homo neanderthalensis, and Homo habilis. Although hominins are related to apes, we have some characteristics that are different. We tend to have larger brains and to stand upright so we can walk on two legs—which is called bipedalism. The fossil record shows that many species of hominins shared with us the ability to communicate complex thoughts and make tools. Other than us, all hominins are now extinct.

So, how similar were our ancestors to us? And where did they all go?

The first humans

The first hominins evolved from the same ancestors as chimpanzees and other modern apes. This common ancestor evolved in two different directions: apes and hominins. The fossil record shows that the first ancient hominins lived in Africa between 4.1 and 1.4 million years ago. These first hominins are known as australopithecines, and they had a mix of ape and human characteristics. Like apes, they had long arms for moving between tree branches. But they also had hip bones and longer legs, which allowed them to walk upright, like we do. They also had smaller teeth than apes, which allowed them to eat a variety of foods. One of their most important differences from apes is that australopithecines had a much larger brain when compared to body size. 

Researchers believe that australopithecines can be grouped into several species, many of which lived on Earth at the same time. These species include:

• Australopithecus africanus

• Australopithecus sediba

• Australopithecus anamensis

• Australopithecus afarensis

• Kenyanthropus platyops
   

By studying bones, researchers have determined that each species had slightly different characteristics. These differences include height, brain size, face shape, and tooth size. 

Even these distant ancestors of modern humans who lived over 3 million years ago seem to have used tools, like we do. Although their tools were very simple, they made it possible for early hominins to gather and eat new types of food. For example, Australopithecus afarensis fossils have been found alongside stone tools that were probably used for carving meat from animal bones and smashing those bones to get at the marrow inside.

A new genus emerges

A fun fact about bone marrow is that it’s high in calories. Researchers believe that early humans’ ability to use tools to get these extra calories from bone marrow may have allowed them to evolve larger and larger brains. The evolution of bigger brains brings us to the next group of our ancestors, the genus called Homo.

At first, all members of the new genus Homo lived in Africa, but their descendants eventually spread to Europe, Asia, and the Americas. As they evolved, their heads became larger, and their faces became flatter. These changes made room for a larger brain. The members of Homo had bones that were fully adapted to bipedalism (walking upright). Like us, they had long legs, curved spines, and arched feet that helped them walk long distances. Walking upright helped early humans watch for both prey and predators and kept their hands free for tool use. These humans also had smaller teeth—well-suited for an omnivorous diet—and hands that were capable of very precise movements.

Until just 40,000 years ago, there were a variety of different species of Homo walking the Earth. The most notable in the fossil record are Homo habilis, Homo erectus, and Homo neanderthalensis (often called Neanderthals). Homo erectus was the first of our genus to use hand axes and fire. This is important because it shows that individuals worked collectively with other members of their social group. Making complex tools and controlling fire are skills that need to be taught and passed on to later generations. 

So, where did all these big-brained, tool-using species of humans disappear to? Why is our species the last one left?

What happened?

Scientists have been studying the evolution of humans since the nineteenth century. Using a wide range of evidence, they have proposed several explanations for why hominins evolved and why Homo sapiens are the only species of humans left today. These explanations include the influence of climate change, evolutionary differences between Homo sapiens and our close relatives, and interbreeding.

From the beginning, natural climate change played an important role in our evolution. Our australopithecine ancestors first evolved in forests in the Great Rift Valley in Africa. About 10 million years ago, the climate there began to dry. Trees died, and jungle biomes turned into grassland. This created new habitats, and gradually, our hominin ancestors evolved traits like bipedalism, which helped them live on the ground instead of trees. Walking upright helped them survive in grassland by allowing our ancestors to look around while walking and to see danger and food further away.

Our hominin ancestors who developed symbolic language and tool use were more likely to be able to work together and survive. This was an advantage not only when climate changed, but when people moved to new areas that had different weather and resources. Over time, later species were better able to use these advantages. Our species, Homo sapiens, were the most successful of all. Our brains were larger in relation to our body. We used more-complex tools. And our advanced abilities to use symbolic language meant we could cooperate in larger numbers and pass on what we learned to later generations. All this together allowed us to migrate out of Africa, adapt to new environments, and spread around the planet, beginning about 70,000 years ago. By about 40,000 years ago, there were almost no other species of humans on Earth.

Why are we alone?

We Homo sapiens are the last human species left. One big reason for this has to do with another instance of climate change. Around 115,000 years ago, the world began to cool. This cooling reached its height around 26,000 years ago. Homo sapiens were probably better able to adapt to this changing environment than some other species of humans. We had more complex tools and a greater ability to communicate. 

However, at least one other species—the Neanderthals—had similar abilities. Where did they go? Part of the answer may have been that their communities were smaller than ours and became isolated in some cases. But the more important answer may be that they didn’t go anywhere. They’re still with us, because we interbred with them. In other words, some of our ancestors were Neanderthals. We know this because our modern Homo sapiens genetics shows that many of us are part Homo neanderthalensis.

So, don’t make fun of Neanderthals. One of them may be your great-great grandmother to the hundredth degree.