The genes that regulate brain development and function evolved much more rapidly in humans than in nonhuman primates and other mammals because of natural selection processes unique to the human lineage. Researchers reported their findings in the cover article of the Dec. 29, 2004, issue of the journal Cell.
"Humans evolved their cognitive abilities not due to a few accidental mutations, but rather, from an enormous number of mutations acquired though exceptionally intense selection favoring more complex cognitive abilities," said lead scientist Bruce Lahn, an assistant professor of human genetics at the University of Chicago and an investigator at the Howard Hughes Medical Institute.
"We tend to think of our own species as categorically different – being on the top of the food chain," Lahn said. "There is some justification for that."
From a genetic point of view, some scientists thought that human evolution might be a recapitulation of the typical molecular evolutionary process, he said. For example, the evolution of the larger brain might be due to the same processes that led to the evolution of a larger antler or a longer tusk. It's just a particular feature that is exaggerated in the human species.
"We've proven that there is a big distinction. Human evolution is, in fact, a privileged process because it involves a large number of mutations in a large number of genes," Lahn said. "To accomplish so much in so little evolutionary time – a few tens of millions of years – requires a selective process that is perhaps categorically different from the typical processes of acquiring new biological traits."
Generally speaking, the higher up the evolutionary tree, the bigger and more complex the brain becomes (after scaling to body size). But this moderate trend became a huge leap during human evolution. The human brain is exceptionally larger and more complex than the brains of nonhuman primates, including man's closest relative, the chimpanzee.
One way to study evolution at the molecular level is to examine changes of when and where proteins are expressed in the body. "But there are many challenges to study the evolution of protein expression. Instead, we chose to track structural changes in proteins," said graduate student Eric Vallender, lead author of the article along with former graduate student Steve Dorus, both of Lahn's laboratory.
Researchers examined the DNA of 214 genes involved in brain development and function in four species: humans, macaques (an Old World monkey), rats and mice. (Primates split from rodents about 80 million years ago; humans split from macaques 20 million to 25 million years ago; and rats split from mice 16 million to 23 million years ago.)
For each of these brain-related genes, they identified changes that altered the structure of the resulting protein, as well as those that did not affect protein structure. Only those genetic changes that alter protein structure are likely to be subject to evolutionary selection, Lahn said. Changes in the gene that do not alter the protein indicate the overall mutation rate – the background of random mutations from which evolutionary changes arise, known as the gene's molecular clock. The ratio of the two types of changes gives a measure of the pressure of natural selection driving the evolution of the gene.
Researchers found that brain-related genes evolved much faster in humans and macaques than in rats and mice. Additionally, the human lineage has a higher rate of protein changes than the macaque lineage. Similarly, the human lineage has a higher rate than the chimpanzee lineage.
"For brain-related genes, the amount of evolution in the lineage leading to humans is far greater than the other species we have examined," Lahn said. "This is based on an extensive set of genes."
They argue that a significant fraction of genes in the human genome were impacted by this selective process. The researchers estimate there may have been thousands of mutations in thousands of genes that contributed to the evolution of the human brain. This "staggering" number of mutations suggests that the human lineage was driven by intense selection process.
The study also revealed two dozen "outliers" – those genes with the fastest evolutionary rates in the human lineage. Of these, 17 are involved in controlling brain size and behavior, arguing that genes that affect brain size and behavior are preferential targets of selection during human evolution. Lahn and his colleagues now are focusing on these outlier genes, which may reveal more about how the human brain became bigger and better.
For two of these outliers, ASPM and Microcephalin, previous work from Lahn's group already has implicated them in the evolutionary enlargement of the human brain. Loss-of-function mutations in either ASPM or Microcephalin cause microcephaly in humans – a severe reduction in the size of the cerebral cortex, the part of the brain responsible for planning, abstract reasoning and other higher cognitive function.
One of the study's major surprises is the relatively large number of genes that have contributed to human brain evolution. "For a long time, people have debated about the genetic underpinning of human brain evolution," said Lahn. "Is it a few mutations in a few genes, a lot of mutations in a few genes, or a lot of mutations in a lot of genes? The answer appears to be a lot of mutations in a lot of genes. We've done a rough calculation that the evolution of the human brain probably involves hundreds if not thousands of mutations in perhaps hundreds or thousands of genes -- and even that is a conservative estimate."
It is nothing short of spectacular that so many mutations in so many genes were acquired during the mere 20-25 million years of time in the evolutionary lineage leading to humans, according to Lahn. This means that selection has worked "extra-hard" during human evolution to create the powerful brain that exists in humans.
Lahn further speculated that the strong selection for better brains may still be ongoing in the present-day human populations. Why the human lineage experienced such intensified selection for better brains but not other species is an open question. Lahn believes that answers to this important question will come not just from the biological sciences but from the social sciences as well. It is perhaps the complex social structures and cultural behaviors unique in human ancestors that fueled the rapid evolution of the brain. "This paper is going to open up lots of discussion," Lahn said. "We have to start thinking about how social structures and cultural behaviors in the lineage leading to humans differed from that in other lineages, and how such differences have powered human evolution in a unique manner. To me, that is the most exciting part of this paper."
Though I am skeptical that better brains are being selected for today. In industrial countries smarter people have fewer kids. So how are better brains still being selected for? I don't see it.
His research and speculations put Lahn into politically incorrect territory. The idea that human brains are still under selective pressure (which is extremely obvious in my view) opens up the possibility that different human environments have been and still are selecting for cognitively different humans. If humans are being selected for to be better suited for each enviromental niche that humans have occupied and currently occupy then humans have less in common with each other than radical egalitarians would like us to believe.
These brain genes that Lahn found to have undergone so many changes leading to humans are excellent candidates for genes that cause changes in intelligence between humans. My guess is that within 10 years DNA sequencing costs will be so low that large scale comparisons of Lahn's genes of interest will be possible using humans with different measured levels of IQ. Then we will find out which genetic variations cause higher levels of intelligence and also different patterns and styles of thinking. For example, I fully expect genetic variations that cause conservative and left-liberal and other categories of political attitudes will be found.
Also see my previous posts on Bruce Lahn's research on human brain genetics and brain evolution: Researchers Find Key Gene For Evolution Of Human Intelligence and Human Brain Size Regulating Gene To Be Inserted Into Mice and Genetic Causes Of Infidelity Found In Twins Study.
Update: Godless Capitalist has more coverage of this story.
|Share |||Randall Parker, 2004 December 30 02:47 AM Brain Evolution|