Science Interviews


Sun, 14th Nov 2004

The Genes That Make Us Human

Dr Armand Leroi

Part of the show Origin of Earth, The Moon and Life

Chris - What are the genes that make us human and separate us from, say, chimps ?

Armand - One of the deepest questions in biology, one that goes back to Aristotle. Namely, what exactly is the difference between humans and animals? Not very much. If you line up the human genome against our nearest relatives - the chimpanzees - you find that the differences are very few indeed.

Chris - What is the correct number?

Armand - The number changes as people investigate the genome in ever greater detail. In the '70s the number was about 98.5%. In the last year, the chimp genome has been sequenced and the human genome was sequenced in 2001. Now you can line them up side-by-side and count differences, and we are about 5% different.

Chris - What does lining up the genome tell us about where the humans came from and when the 2 split apart? Can we put a time point to when humans began to exist independently of the monkeys?

Armand - We still have a date of about 6 million years. The genome doesn't tell us that much directly about the date. You can use it to date divergences, but the best evidence still comes from the fossil record.

Chris - Is it fairly plausible that mankind originated in Africa about 3 million years ago?

Armand - That certainly looks the way it is from all the molecular and fossil evidence.

Chris - How are you using genetics to pinpoint what makes us human? What is the 5% that splits us away from chimps?

Armand - The problem is not that we're so similar, The problem is that we're so different. Our genomes are about 3 billion DNA building blocks long. If we are even only 1% different, that's 30 million nucleotides. That's a huge number of differences.
Chris - How are you going to be able to pinpoint which of those differences are the important ones?

Armand - One way is by looking at human mutants, specifically at mutants that tell us about parts of our body that are most different from chimpanzees, and most specifically our brains.

Some work done by Jeff Woods in Leeds studied one of these mutations. There is a family of people with very small heads - microcephaly -heads 3 standard deviations smaller than mean. There is in Pakistan a city where microcephaly is very common.

This disorder is due to a recessive mutation, which means that to get this disorder you need 2 copies of the bad gene. The reason you actually get the disorder is because of the Pakistani habit of consanguinity - when people tend to marry their cousins. Since they are part of the same family, each cousin may have 1 copy of the bad gene, but their children might get 2 copies.

This gene controls the rate the brain actually grows. If you damage that gene by mutation you get a small head. You can take this gene and compare it to the homologous (similar) gene in chimpanzees. Although it's very similar, it's still much more different than most genes. In other words, this gene has been evolving fast over the past 2-3 million years.

Chris - Do you think it's just big brains that set us apart from chimps?

Armand - Big brains can't be the only things that make us special. Genes that are involved in the structure of the brain have been changing simultaneously. Another gene - Phoxp2 - when mutated in humans causes various kinds of linguistic and grammatical deficiencies. You have this gene in chimps, but when you compare it to humans, there are all sorts of differences. It has been evolving really fast, and the reason ithas been evolving is because it is one of the gene that gives us language

Yvonne (Caller): Dinosaurs were on the earth for 9 million years. I have never heard of any developing a very high intelligence. Why not?

Armand - The short answer is I don't know. It's always difficult when explaining unique historical events to give an account why one group has evolved high intellegence. There are trends towards bigger brains in dinosaurs. You find this especially in the big predator lineage.


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