An introduction to biological systematics
An introduction to biological systematics

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

An introduction to biological systematics

2.11 Consequences of human / chimp pairing

Activity 10

Timing: 0 hours 10 minutes

This clip refers back to the table of molecular characters, which is shown again here (Figure 9). It explains the consequences of human / chimp pairing in terms of homologous and non-homologous characters.

Figure 9 Nucleotides at selectived positions (left column) in sequences of non-codingg DNA in the region of the beta haemoglobin family of genes in various higher primates. Asterisks denote gaps in the sequences of the species concerned. Based on Williams, S. A. and Goodman, M. (1989) A statistical test that supports a human/chimpanzee clade based on non-coding sequence data, Mol. Biol. Evol., 6, 325-330
Download this audio clip.Audio player: Audio clip 11
Skip transcript: Audio clip 11

Transcript: Audio clip 11

Dr. Colin Patterson
Now accepting this human / chimp pairing has various consequences. One is that it means that characters uniquely shared by humans and chimps - like the twelve nucleotides in the table, or the fusion of premaxilla and maxilla in the adult - are homologous. But characters uniquely shared by chimps and gorillas, or by humans and gorillas, aren't necessarily non-homologous.
Look at character 9 in the table, position 5156, where chimp and gorilla share Guanine. We can explain that in two ways. Either the Guanine was independently acquired in chimp and gorilla by two separate mutations from the ancestral Adenine, or Guanine was acquired by a mutation in the common ancestry of human, chimp and gorilla, and then humans reverted to adenine by a second mutation. Each explanation requires just two mutations, so they're equally parsimonious. But the explanation by independent mutations in chimps and gorillas says that the two Guanines are non-homologous – they’re convergent or chance similarity.
The other explanation, a mutation in the common ancestry of human, chimp and gorilla, says that the two Guanines are homologous, but are primitive for apes, just as Adenine is primitive at this site for the whole group in the table. This same sort of argument works for morphology. I said, a few minutes ago, that chimps and gorillas share various features of the hand and arm, associated with knuckle-walking. Now, given that chimps are most closely related to us, we can explain that in two ways. Either chimps and gorillas independently acquired knuckle-walking, or it developed in the common ancestry of humans and African apes, and was then lost in the human line. In the first case, the features are non-homologous in chimps and gorillas, but were acquired by parallel evolution. And, in the second case they are homologous. In this instance, we might guess that knuckle-walking is too complex to develop twice in exactly the same way, so it's probably primitive for the group, and is lost in us. Or, we could guess that knuckle-walking is obviously adaptive, and might well develop by natural selection, independently in two closely related lines.
I think all you need remember from this is that homology is a conclusion we infer from a tree or a cladogram. It's not something we can establish directly. And then that there are two kinds of homologies, derived ones and primitive ones. But every homology has to fit on the tree somewhere, as a derived character, a synapomorphy of a group.
End transcript: Audio clip 11
Audio clip 11
Interactive feature not available in single page view (see it in standard view).

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371