Cognitive Science: An Introduction/A Brief History of Human Evolution

Basic Timeline

To the best of our knowledge, our universe began about 13.5 billion years ago. Earth formed 4.5 billion years ago, and the first organisms came into existence 3.8 billion years go. These organisms were very simple, but self-replicating, which allowed evolution to take place.

The first two billion years of evolution created creatures that could maintain themselves, acquire energy from the environment, and reproduce. These were single-celled creatures. Somehow one creature swallowed another and they ended up helping each other--that's how we got our mitochondria, the powerhouses of our cells.

Half a billion years ago evolution on Earth had the "Cambrian Explosion," in which a huge diversity of new life forms evolved. Mammals and dinosaurs existed at the same time, but mammals really got a foothold on ecosystems after the Cretaceous-Paleogene extinction, sixty-six million years ago.

The last common ancestor of both humans and chimpanzees lived about 6 million years ago. The scientific name for our species is Homo Sapiens. We are the only species of Homo left on Earth, but there were several others that have gone extinct. Members of the genus Homo appeared about 2.5 million years ago. There were lots of humans and human-like beings on Earth that are not our ancestors, genetically speaking. They died out.

Homo Neanderthalensis, the Neanderthals, evolved 500,000 years ago, in Europe and the Middle East. Members of Homo were using fire on a daily basis 300,000 years ago. In east Africa, Homo Sapiens evolved 200,000 years ago--note that this is far later than the first Homo species. Homo Sapiens started using language about 70,000 years ago. This time can justifiably be thought of as the dawn of high-level cognition on Earth. In the period between 70,000 and 30,000 years ago human beings started making boats, art, oil lamps, bows and arrows, needles, religion, social stratification, and commerce.

The Lioness-woman figurine from the Stadel Cave in Germany, from about 32,000 years ago. It's one of the first examples of art, and probably religion. Because it depicts something that doesn't exist (a person with a lion head), it demonstrates the ability to imagine.

Speculation on what a Neanderthal might have looked like, based on the skull fragment fossils we have found.

45,000 years ago "sapiens" settled in Australia, and promptly made extinct all of the very large animals that lived there (megafauna)

30,000 years ago the Neanderthals went extinct, though modern "Sapiens" have Neanderthal genes (about 1%-4% for Europeans and people of the Middle East), so there was interbreeding.

16,000 years ago sapiens entered the Americas, and promptly made extinct all of the megafauna that lived there. 13,000 years ago Homo floresiensis went extinct, leaving Homo sapiens as the last surviving species of the genus Homo.

Every time humans crossed an ocean, there were only very few people who survived to the other side. This caused a genetic bottleneck. That is, most people in the new world descended from the few people that arrived there. This means that there is less genetic diversity in the new world than in the old. So there is more genetic diversity in Africa than in Asia, and more in Asia than in Australia, because each of these moves caused a genetic bottleneck. A similar process happens with language and accent differentiation. So, by studying genetic (and linguistic) diversity in different geographical areas, we can get a picture of where humans came from.

12,000 years ago was the agricultural revolution, the domestication of animals and plants, and the beginning of permanent settlements. 500 years ago (around 1516) was the scientific revolution.

Grasping the Vast Timescales of Evolution

It’s difficult to visualize big numbers. but without something to visualize, they are really hard to appreciate. For example, simple cells formed 3,600,000,000 years ago. That’s three billion, six hundred million. How can we picture that?

Swimming Pool

Swimming pools are something people have some experience with. If the water in a pool represented the time since the beginning of life, when simple cells first developed, we can look at more recent events and ask how much pool water it takes to represent the amount of time since they happened. We can do some quick calculations to give us a feeling for the vastness of time.

If an average pool holds 375,000 liters of water, human evolution diverged with chimpanzee evolution between 5 and 7 million years ago, which is about 520 liters of water. That's about two big bathtubs.

Anatomically modern humans appeared about 200,000 years ago, which is only 20 liters of water, or about 10 two-liter bottles of soda's worth.

The dawn of agriculture was about 10,000 years ago, which is just one liter or water.

A human lifespan of 80 years is a mere two teaspoons of water in the great big pool of life.

Genes

Genes are stored in molecules called DNA. There is a copy of your genes in every cell of your body. DNA is a string about two meters long, but coiled. If you were to scale up DNA so that it was the thickness of a piece of sewing thread, it would be about 200 kilometers long. Picture that--two meters of DNA in every cell. Think about stuffing headphones in your pocket, and how knots form. DNA can't have any knots, because to use the genes in it it needs to be able to unfold at any spot along the chain.

The Environment of Evolutionary Adaptation (EEA)

When thinking about human minds and how they evolved, it's easy to look at our current technological environment. But we spent much more time as bands of about 150 hunter-gatherers in the Pleistocene era. Most evolutionary psychologists believe that most of our evolved psychological traits were formed during this time period, which they call the environment of evolutionary adaptation, or the EEA. This was a long time ago, so we have to use indirect methods to know what life was like then.

One way is to look at contemporary hunter-gatherer societies, and assume (without specific reasons not to) that our EEA counterparts were similar.

Contemporary Hunter-Gatherer Societies

Looking at one particular tribe, the Hadza, we can expect the average person to live into their thirties. But this is only an average--about one-fifth of hunter-gatherer children die before they reach one year of age, and close to half die before they reach adulthood. About 1% of mothers die in childbirth. But there are plenty of people who live into their seventies or even their eighties. If someone is lucky enough to live to be 45, they can expect to live another 21 years.

For example, a study of 32 contemporary hunter-gatherers showed that 25% of the people one meets are relatives or distant kin, about 50% are related to one through marriage (including marriage of one' siblings), and only 25% of the people one ever meets are completely unrelated.

History of Technology

One way to look at human intelligence evolution is to look at when we started making arts and tools. This is a continually-changing field, because finding really ancient artifacts, though rare, keeps happening. No doubt in the future we find evidence of tools being made even earlier than those we know of today. One problem with this field is that the only artifacts that are preserved from long ago are those made of durable materials--mostly stone. All organic tools, including clothing, anything made of wood, and most pottery, won't leave any trace at all. If we look at the tools of contemporary hunter-gatherers, we can see that most of their tools are made of organic material. The "stone age" should be called the "wood age." Suppose humanity had a natural disaster that destroyed all evidence of civilization except for styrofoam. Alien anthropologists might call the 21st century the "foam age."

There is evidence of specially-created bedding dated at 77,000 years ago, using leaves that had natural insecticides in them. She also found evidence of glue used to hold wood together (perhaps for shelters.) Going further back, we have evidence that people heated stone over a controlled heat to make a modifiable material 164,000 years ago.

Creativity of some kind seems to pre-dated humanity altogether. There is evidence of neantertals making glue 200,000 years ago. Going even further back, Homo heidelbergensis (the last common ancestor of Homo sapiens and Neandertals) formed lethal spear tips 500,000 years ago. Going a million years back, Homo erectus kindled fires for protection and warmth. However, before Homo sapiens, innovation needed genetic evolution to happen. Homo erectus, for example, made the same kind of stone tools for two million years! Compare this to the rapid change of techological evolution we see in Homo sapiens.

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