In recent decades, scientists have discovered more about the beginnings of humanity. The fossil record shows a gradual transition over 5 million years ago from chimpanzee-size creatures to hominids with larger brains who walked on two legs. Later hominids used fire and stone tools and had brains as large as modern humans. Fossils of homo sapiens in east Africa date back nearly 200,000 years. Humans developed hearths for fire, stone points for spears and arrows, and cave paintings by 30,000 years ago. By 10,000 years ago, humans had spread throughout the globe. Genetic studies support the same picture. Humans share more DNA with chimpanzees than with any other animal, suggesting that humans and chimps share a relatively recent common ancestor. Also, the same defective genes appear in both humans and chimps, at the same locations in the genome—an observation difficult to explain except by common ancestry. Genetics also tells us that the human population today descended from more than two people. Evolution happens not to individuals but to populations, and the amount of genetic diversity in the gene pool today suggests that the human population was never smaller than several thousand individuals. Yet all humans, of all races, are descended from this group. Humanity is one family.
In the last couple of decades, our understanding of genetics has grown dramatically, providing overwhelming evidence that humans share common ancestors with all life on earth. Here are some of the main types of genetic evidence for common ancestry.
1. Genetic Diversity. Human children inherit 3 billion base pairs of DNA from each parent, but they are not an exact duplicate. The rate of change has been measured precisely to an average of 70 bases (out of our 6 billion total) per generation. So as we go back on the family tree, there are more and more genetic differences between us and our ancestors. For example, there would be about 140 differences between your DNA and that of your four grandparents, and 210 differences between you and your eight great-grandparents, and so on. That enables us to make a prediction from the amount of genetic diversity between two species about the time since their common ancestor population lived. Using non-genetic evidence, the common ancestor between humans and chimpanzees was estimated to have lived about 6 million years ago. The calculation from genetic differences gives a figure remarkably close to the estimated value.
2. Genetic “scars”. Just as scars stay on our bodies as reminders of past events, the DNA code contains “scars” and these are passed on from generation to generation. DNA scars result from the deletion or insertion of a block of bases (not just single base changes as in the previous section). Because we have a lot of these (hundreds of thousands) and they can be precisely located, they serve as a historical record of species. If we have the same scar as chimpanzees and orangutans, then the deletion or insertion must have occurred before these species diverged into separate populations. If we and chimpanzees have a certain scar but orangutans do not, we can conclude the deletion or insertion must have occurred after the common ancestor of chimps and humans separated from our common ancestor with orangutans. In this way we can create a detailed family tree of common ancestors.
3. Genetic synonyms. In a certain context, the words “round” and “circular” mean the same thing to an English speaker—they are synonyms. So too, there are “synonyms” in the genetic code—different sequences of DNA bases that mean the same thing to cells (that is, they cause the production of the same proteins). Mutations in the genetic code are often harmful, resulting in an organism not being able to successfully reproduce. But if the mutation results in a “synonym”, the organism would function the same and continue passing on its genes. Because of this we would expect the synonymous changes to be passed on much more effectively than non-synonymous changes. That is exactly what we find among the DNA of humans and chimpanzees: there are many more synonymous differences between the two species than non-synonymous ones. This is exactly what we would expect if the two species had a common ancestor, and so it provides further evidence that humans and chimpanzees were created through common descent from a single ancestral species.
The more research that is done on DNA, the more evidence we find that all life is related.