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Battles over teaching evolution may be playing out near you. Meanwhile, scientists have new evidence that our most important organ — the brain — is still evolving. This ScienCentral News video has more.
Migrating Mutations
With, or without us, our world continues to change at an incredible pace. The twentieth century has seen cars getting closer and closer to the supposed futuristic fantasies of the manufacturers of the 1950's; computers that, less than 40 years ago filled a whole office, now sit nicely on each of our desks; scientists are making breakthroughs in fighting diseases that only decades ago hadn't even been identified yet.
In order to keep pace with our ever-changing environment we all have to learn to adapt, and as the father of the theory of evolution Charles Darwin declared, adaptation can mean survival. So as a race are we still evolving? It seems that we are.
Researchers have shown that mutations, or variants, in two genes thought to regulate brain size have only arisen recently in the long history of evolution of the planet and appear to be spreading quickly in large swaths of the world's population. Armed with this fresh evidence, one researcher argues that our brains are still evolving.
"We've caught evolution in action, in the sense that here is a new variant in each one of these two genes that arose very recently," says human geneticist Bruce Lahn, from the University of Chicago and an investigator at the Howard Hughes Medical Institute. "Our findings just added another piece of evidence to the idea of evolution, that as a species we're still evolving, even as a very complex species we're still evolving. We can see that at the level of genes, that is, our genes are still in the process of changing."
Specifically, Lahn looked at two genes — microcephalin and abnormal spindle-like microcephaly-associated (ASPM) — both known to regulate brain size during development. "If either one of these two genes is mutated causing a loss of function in the gene, it results in a developmental defect in the brain, such that the brain size becomes about a third of the normal range," says Lahn. "We figured that if these genes are required for large brain size in development then they might have also contributed to the enlargement of the brain during evolution."
After analyzing gene samples from nearly 1,200 people worldwide, Lahn's group found that the new mutation class of microcephalin emerged about 37,000 years ago and now shows up in about 70 percent of people in most European and East Asian populations, and almost 100 percent of those in three South American Indian populations, but is much rarer in most sub-Saharan Africans. Meanwhile, the ASPM mutation only popped up about 5,800 years ago, but has since spread to approximately 30 percent of humans today, with the highest occurrence appearing in people of the Middle East and Europe, but less so in East Asia, sub-Saharan Africa populations.
"Microcephalin corresponds roughly to the time of coexistence between humans and Neanderthals in Europe," Lahn says. "ASPM corresponds to the beginning of agriculture, about 10,000 years ago, and the first evidence of written language and also several cities in the Middle East, about 6,000 years ago."
In 2004, Discover.com reported on earlier research by Lahn and his collaborators, who observed that 20 brain-associated genes, including microcephalin and ASPM, evolved faster in the great ape lineage than in mice and rats. He concluded that these genes may have played important roles in evolving the human brain. "The evolution of these two genes was dramatically accelerated in humans compared with other non-human primate lineages or other non-primate mammals," Lahn says. "So we suspected the evolution of these two genes might have to do with human brains evolving to become larger over evolutionary time."
image: NIH
According to Darwin's theory, evolution doesn't occur at the species level, rather, when our DNA is replicating itself within our bodies, sometimes errors or variants occur. If that error results in a greater likelihood of survival in people with the mutation, the variant spreads in the population. Mutations are "still spreading in humans because people who have these variants are slightly more fit and leave behind more children than people who don't," Lahn explains.
He says that natural selection works a lot like this: if a manuscript written 2,000 years ago was transcribed again and again into many copies, over many generations, during transcription errors are continually introduced so that all modern copies are not identical to each other. Likewise with our genes, people in different parts of the world have copies with different errors.
Mutations in these genes first came to light in earlier research because variants can result in a disease called microcephaly. People with the condition are born with a brain that is much smaller than usual, often with a substantial shrinkage of the cerebral cortex that seems a throwback to when the human brain was a fraction of present size. "But what's remarkable about patients with mutations in these genes is that besides having a small brain, they don't seem to have abnormalities anywhere else, which implies that these two genes are very specific regulators of brain size," Lahn says.
But Lahn stresses that he's not saying these genes make our brains bigger, or us any smarter: "We don't really know what these new variants do. Even though they confer some type of selective advantage such that they spread in human populations, that doesn't mean that they necessarily make people smarter. There's evidence that on average bigger brains make people a little smarter but there are a lot of people who have small brains who are smart and there are a lot of people who have big brains who are not smart."
Lahn adds that the brain is so complex that other genes likely also play a role in helping our brains continue to evolve. He is currently looking at mutations in other brain genes, and at the same time is trying to figure out if the mutations in ASPM and microcephalin play a role in warding off brain diseases.
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