Common Ancestors

Key to understanding evolutionary history is to understand the concept of a common ancestor. If you ask students whether chimpanzees are descended from humans, they will laugh and tell you, “Of course not.” But if you ask them whether we are descended from chimpanzees, or even monkeys, they will not be too sure. In fact, the popular demonization of evolution often takes the form of “My great-grandfather wasn't a monkey.” Here is where the tree comes in. The tree shows that ~6 million years ago (mya), humans and chimpanzees had a common ancestor. That common ancestor was not a human and not a chimpanzee. It was no more a chimpanzee than it was a human. We are not descended from chimpanzees. We had a common ancestor with the chimpanzee ~6 mya. Similarly, we had a common ancestor with the gorilla between 7 and 8 mya, with the orangutan ~12 mya, with the gibbons ~18 mya, with the macaque monkey ~25 mya, and with the mouse ~75 mya. Because these common ancestors are physically presented in the tree, it is easier for students to understand their significance.

In a sense, these common ancestors are hypothetical, since it is difficult to identify them with certainty from the fossil record. And while I do not discuss the details of how we know the divergence dates, they are estimated from a combination of the fossil record and sophisticated computer programs.

Students can be presented with an exercise in which they are asked to identify when the common ancestors of any two of the organisms on the tree lived. They can then be asked to infer from this which organisms are more closely related to each other. Such a sample activity is shown in Figure 2.

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Figure 2.

Sample activity for students based on the primate tree. You can add your own questions. Students can be given a table that lists organisms only and be asked to fill in the time of the most recent common ancestor. Other combinations of organisms can be used, and you can write additional questions about which organisms are more closely related to each other. The basic principle for students to understand is that the more recently two organisms had a common ancestor, the more closely related they are to each other.

Students will want to know what this common ancestor of humans and chimpanzees was like. We don't really know all the details, but there are some clues in the fossil record. Ardipithecus ramidus, nicknamed “Ardi,” was a hominin that lived 4.4 billion years ago (Gibbons, 2009). It is the oldest hominin that has been characterized in great detail and gives us some idea of what this common ancestor might have been like. Ardi had a small brain (~400 cc), like chimpanzees. But Ardi had a foot that, except for an opposable big toe (like the chimpanzees), is much more like a human foot than a chimpanzee foot. So, in some ways (brain size), this common ancestor of the human and chimpanzee was more like a chimpanzee, while in other ways (foot architecture), it was more like a person. And that's the point. It was not a chimpanzee or a human. It was the common ancestor of both. For those wishing to explore our more immediate ancestors in more detail, there are many projects that students will enjoy. The Gibbons (2009) reference is an excellent place to start. You can get pictures of Ardi's skeleton, and detailed descriptions of many of Ardi's characteristics. If your school is near a natural history museum with exhibits of early hominin ancestors, students will find a field trip to be an exciting and unforgettable experience.

Humans have 46 chromosomes, whereas the chimpanzee, gorilla, and orangutan all have 48. The most likely explanation is that the common ancestor of the human, chimpanzee, gorilla, and orangutan had 48 chromosomes, and there was a fusion on the lineage leading to humans, after humans and chimpanzees had diverged from their common ancestor. This has been confirmed with karyotype analysis and DNA sequencing. It has long been known that human chromosome 2 looks like two smaller chimpanzee chromosomes fused together. And DNA sequencing shows the presence of telomeric sequences near the centromere of human chromosome 2, confirming that these were once ends of smaller chromosomes.

The tree can also be used to discuss the idea of introgression, the sharing of genes between closely related species. Although the common ancestor of humans and chimpanzees lived ~6 mya, detailed studies of their genomes show that they continued to exchange genes with each other as recently as 5 mya (Patterson et al., 2006). This allows us to discuss some of the latest thinking on speciation. Speciation is a process, not an event (Grant & Grant, 2008, 2014). It can take a long time, up to millions of years, after a population splits into two lineages before the new lineages cannot ever reproduce with each other. The Grants observed this in the Galápagos finches. Under conditions of duress, finches of different but closely related species did reproduce with each other and have fertile offspring. In the case of humans and chimpanzees, the introgression was picked up by sophisticated computer programs that were able to compare the high-quality DNA sequences of the two species.

This tree raises many other questions. For example, what are the species that lived after our lineage separated from the chimpanzees? These would be in the space between the human and chimpanzee lineages – a topic for another paper.

It takes years to be proficient at reading phylogenetic trees. I sometimes struggle with the more complicated ones. We do not have that much time with our students. However, by getting them used to accurately reading simple trees, we will have given them an important skill that will make it easier for them to use phylogenetic trees in their thinking, and to eventually be proficient in reading the more complicated trees they may encounter.