Phylogenetic evidence Hangin round on the Tree of Life

As I explain in Chapter 9, phylogenetics takes data about existing species and reconstructs the evolutionary branching pattern that led to those species. Not surprisingly, no small amount of effort has been devoted to reconstructing the parts of the tree of life where humans reside. Our particular branch includes the apes: gibbons, orangutans, gorillas, the two types of chimpanzees (the standard one that you're familiar with and the bonobo, which used to be called the pygmy chimp but which turns out to be a species of its own), and us!

When biologists started wondering where all these creatures should reside on our branch of the tree, they imagined one sub-branch leading to us and another sub-branch leading to all those charming, furry creatures who seem so different from us. Researchers could tell that, of all the animals, humans are most like apes, but in the past, they tended to think of the apes as belonging on the other twig of our shared branch of the tree.

Enter the amazing resource of DNA. Now that scientists have been able to sequence human DNA as well as good samples of DNA from the other apes (in the case of the chimpanzees, the entire genome), they've discovered that humans and chimps aren't very distant at all. The current best hypothesis about the relationships between humans and the rest of the great apes is shown in Figure 16-1.

In Figure 16-1, you can see that the two chimpanzee species (chimps and bonobos) have a most recent common ancestor. These species are a lot more like each other than they are like anything else. But — surprise! — humans have a most common ancestor with the chimpanzee lineage that we don't share with the other great apes. To find the most common ancestor of gorillas, humans, and chimpanzees, you have to go back a bit farther — and farther yet to find the most recent common ancestor of orangutans, gorillas, humans, and chimpanzees.

Figure 16-1:

The relationship between humans and other apes.

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2-3 million years ago

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17-20 million years ago

Figure 16-1:

The relationship between humans and other apes.

Figure 16-1 shows a point that I want to drive home: Humans didn't evolve from chimps! Instead, chimps and humans arose from the same common, apelike ancestor.

Kissing cousins

Humans turn out to be a lot more similar to chimpanzees than biologists first thought. Human DNA is about 95 percent the same as chimpanzee DNA. A lot of active research is going on in this area, and some studies put the number at 97 percent or 98 percent. Whatever the precise percentage, however, the bottom line is that humans are very similar to chimps.

Why do the different percentages exist? Because it's no easy task to figure out which bits in the DNA sequence are genes, which bits might be genes, and which bits are just junk. (Yes, believe it or not, humans, chimps, all other mammals, and most multicellular creatures have a lot of junk DNA; refer to Chapter 15 to find out more about non-coding DNA.) As a consequence, different researchers come up with different estimates regarding which parts are genes and which parts aren't, and thus different estimates of similarity between species.

By combining the information about the relative differences in the genomes of chimpanzees and humans with what we know about the rate of DNA substitutions in specific genes in these two organisms, researchers can tell that, approximately 5 million to 7 million years ago, the lineage leading to modern humans split from the lineage leading to modern chimps. (For information on how to use DNA as a molecular clock to determine the time in the past when two lineages split from a common ancestor, head to Chapter 15.)

^jjjABEft If the DNA sequence of humans is so close to that of the chimpanzee, why do the two species look so different? They certainly don't seem to be only 5 percent different (at least, that's what we humans like to think!). The answer is that small changes in regions of the DNA that have a regulatory function can have major effects, as explained in Chapter 14, including examples of genes that may be important in the different developmental trajectories of chimps and humans.

You say hominid; I say hominian

Biologists commonly name every group; everything that has a common ancestor gets a name. And because humans (the species Homo sapiens) are the ones who do the naming, we've made sure that every higher group that includes us also starts with the letters hom — hominid; hominine; hominin (yes, it means something different from hominine); hominian; and, of course, human.

These names come up a lot in the published studies of human evolution, and they certainly do sound awfully scientific. My opinion? They're nothing but trouble. So in this book, I don't use any of these terms except hominid, which refers to all the creatures on the branch of life starting at the common ancestor of chimps and humans, and leading up in time along the human branch. If you need a more precise definition, try this one on for size:

HOMINID: Modern humans and their extinct relatives, going back to the most common recent ancestor with the chimpanzee lineage.

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