by Greg Mayer
In a soon to be published paper in the Royal Society’s Biology Letters (abstract only), Carles Lalueza-Fox of Universitat Pompeu Fabra ( website in Catalan!) in Barcelona and colleagues report that they have sequenced the gene TAS2R38 from a Neanderthal man (press coverage by the BBC and NY Times). The ability to sequence genes from fossil material is remarkable enough in itself, but this study has particular interest, and not just because it was done on one of our fossil relatives. Variation in the gene they sequenced is responsible for the polymorphism in modern man for the ability to perceive bitter tastes (some people can tast bitter, some can’t). Determining the frequency of the two forms (or alleles) of the gene is a classic high school biology exercise, carried out by seeing who can taste the bitter chemical PTC. People who have either one or two copies (humans are diploid, so most genes are present in each individual’s genome in two copies) of the taster allele can taste bitter; those with two copies of the non-taster allele cannot. Today, the two alleles are about equally frequent, so that about 25% of people have two taster alleles (i.e. they are homozygous for the taster allele), about 50% have one taster and one non-taster (they are heterozygotes), and 25% are homozygous for the non-taster allele.
The Neanderthal they sequenced was a heterozygote, and thus could taste bitter (and also [with sample of only 1, mind you] had the same allele frequencies as we do). The polymorphism thus goes back somewhere on the order of 40,000 years. But Neanderthals split from the lineage leading to modern humans on the order of 300,000 years ago, with little or no subsequent interbreeding. So the polymorphism probably goes back even further, predating the modern Homo sapiens/Neanderthal split. Although an exciting find, this is not a record for the antiquity of a modern polymorphism: some are known to predate the human/chimp split (abstract only), and that’s millions of years ago.
Should not our tastes be explained by natural selection?
How does developing a taste for bitterness help one survive? Or is the explanation genetic drift?
Many plants contain compounds that can damage an animal that eats them, and some of these compounds taste bitter, so the ability to taste bitter can help you (i.e. the animal) avoid ingesting, or over-ingesting, potentially toxic compounds. The interesting thing is that about 25% of people can’t taste bitter, when it might seem advantageous to be able to taste it. This observation, by itself, would be compatible with the idea that these two alleles are subject to genetic drift. The fact that the polymorphism has persisted for perhaps hundreds of thousands of years strongly argues that selection, not drift, is responsible, and that furthermore, it is some form of balancing natural selection that maintains both alleles in the population. One form of selection that would do this is if the heterozygotes had the highest fitness (perhaps if they could taste bitter, but were not so repulsed by it that they would not eat bitter foods even during a scarcity). I don’t know of any work that’s been done on the fitness of the three genotypes, but it would be a bit outside the literature I routinely follow.
GCM
It is wonderful to see the scientific method implemented in this way and have it show meaningful results.
Goo question, Andrew Alexander.
Tasty lanugo.
Won’t it be exciting to hear the ID explanation for this, once censorship of their ideas is overthrown?
Oops, I just remembered that the answer is god did it. I can see why such a meaningful explanation has to be censored. Everyone will someday be saying “Why didn’t I think of that?”
Glen Davidson
http://tinyurl.com/mxaa3p
What’s the deal with the other apes? Do they have the same or analogous alleles? Can they taste bitter?
I had no idea that some people couldn’t taste bitter. I thought it was universal.
I’ve always heard that exactly because of the bitterness beer and olives are two foodstuffs that one need to learn to like.
(And of course some people say that broccoli is trying to scare off predators with its foul taste.)
You have to go to the abstract:
It’s just the one chemical, and is a well-known essentially Mendelian trait.
There may be some people who can’t taste bitter at all, but nearly all can. Not tasting phenylthiocarbamide as bitter is not known to be non-adaptive, at least not so far as I know.
Glen Davidson
http://tinyurl.com/mxaa3p
Exciting study! Thanks for keep us up-to-date.
Greg: (sorry, a pedantic statement follows) Technically DNA can’t be extracted from “fossil material” because the bone is no longer there, it is just an impression of the bone. Ancient DNA researchers extract DNA from sub-fossil bones and the bone matrix is still somewhat intact. The best condition for the survival of DNA is cold and dry. Under the best possible conditions, DNA has an theoretical life expectancy of about 100,000 years (the Jurassic Park story would be impossible). Sometime after about 30-40,000 years the DNA fragments are, in most cases, far too short to provide useful DNA sequences.