Human Brain Evolution Was a 'Special Event'

Howard Hughes Medical Institute ^ | 29 December 2004 | Staff

[PatrickHenry]

Genes that control the size and complexity of the brain have undergone much more rapid evolution in humans than in non-human primates or other mammals, according to a new study by Howard Hughes Medical Institute researchers.

The accelerated evolution of these genes in the human lineage was apparently driven by strong selection. In the ancestors of humans, having bigger and more complex brains appears to have carried a particularly large advantage, much more so than for other mammals. These traits allowed individuals with “better brains” to leave behind more descendants. As a result, genetic mutations that produced bigger and more complex brains spread in the population very quickly. This led ultimately to a dramatic “speeding up” of evolution in genes controlling brain size and complexity.

“People in many fields, including evolutionary biology, anthropology and sociology, have long debated whether the evolution of the human brain was a special event,” said senior author Bruce Lahn of the Howard Hughes Medical Institute at the University of Chicago. “I believe that our study settles this question by showing that it was.”

Lahn and his colleagues reported their data in a research article published in the December 29, 2004, issue of the journal Cell.

The researchers focused their study on 214 brain-related genes, that is, genes involved in controlling brain development and function. They examined how the DNA sequences of these genes changed over evolutionary time in four species: humans, macaque monkeys, rats, and mice. Humans and macaques shared a common ancestor 20-25 million years ago, whereas rats and mice are separated by 16-23 million years of evolution. All four species shared a common ancestor about 80 million years ago.

Humans have extraordinarily large and complex brains, even when compared with macaques and other non-human primates. The human brain is several times larger than that of the macaque — even after correcting for body size — and “it is far more complicated in terms of structure,” said Lahn.

For each gene, Lahn and his colleagues counted the number of changes in the DNA sequence that altered the protein produced by the gene. They then obtained the rate of evolution for that gene by scaling the number of DNA changes to the amount of evolutionary time taken to make those changes.

By this measure, brain-related genes evolved much faster in humans and macaques than in mice and rats. In addition, the rate of evolution has been far greater in the lineage leading to humans than in the lineage leading to macaques.

This accelerated rate of evolution is consistent with the presence of selective forces in the human lineage that strongly favored larger and more complex brains. “The human lineage appears to have been subjected to very different selective regimes compared to most other lineages,” said Lahn. “Selection for greater intelligence and hence larger and more complex brains is far more intense during human evolution than during the evolution of other mammals.”

To further examine the role of selection in the evolution of brain-related genes, Lahn and his colleagues divided these genes into two groups. One group contained genes involved in the development of the brain during embryonic, fetal and infancy stages. The other group consisted of genes involved in “housekeeping” functions of the brain necessary for neural cells to live and function. If intensified selection indeed drove the dramatic changes in the size and organization of the brain, the developmental genes would be expected to change faster than the housekeeping genes during human evolution. Sure enough, Lahn's group found that the developmental genes showed much higher rates of change than the housekeeping genes.

In addition to uncovering the overall trend that brain-related genes — particularly those involved in brain development — evolved significantly faster in the human lineage, the study also uncovered two dozen “outlier” genes that might have made important contributions to the evolution of the human brain. These outlier genes were identified by virtue of the fact that their rate of change is especially accelerated in the human lineage, far more so than the other genes examined in the study. Strikingly, most of these outlier genes are involved in controlling either the overall size or the behavioral output of the brain — aspects of the brain that have changed the most during human evolution.

According to graduate student Eric Vallender, a coauthor of the article, it is entirely possible by chance that that two or three of these outlier genes might be involved in controlling brain size or behavior. “But we see a lot more than a couple — more like 17 out of the two dozen outliers,” he said. Thus, according to Lahn, genes controlling the overall size and behavioral output of the brain are perhaps places of the genome where nature has done the most amount of tinkering in the process of creating the powerful brain that humans possess today.

There is “no question” that Lahn's group has uncovered evidence of selection, said Ajit Varki of the University of California, San Diego. Furthermore, by choosing to look at specific genes, Lahn and his colleagues have demonstrated “that the candidate gene approach is alive and well,” said Varki. “They have found lots of interesting things.”

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.

Varki points out that several major events in recent human evolution may reflect the action of strong selective forces, including the appearance of the genus Homo about 2 million years ago, a major expansion of the brain beginning about a half million years ago, and the appearance of anatomically modern humans about 150,000 years ago. "It's clear that human evolution did not occur in one fell swoop," he said, "which makes sense, given that the brain is such a complex organ."

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.”

[Dataman]

The Peppered Moth is not a hoax.

Pay special attention to point #4

Are Darwin's finches also hoaxes?

Insofar as they are presented as proof of evolution. They only illustrate variation.

As for the flood, it supposedly lasted 40 days and nights,

Closer to a year.

That takes millions of years. Are you telling me that all those deposits formed in forty days?

How long do you think these layers took to form?

They formed in a matter of hours.

If the fossils where caused by the Biblical flood then they would all be in one layer, which they are clearly not.

Creationists don't claim this. Your statement assumes the flood layer would be just one layer. See the picture above. That was all laid down by the same flood. Don't forget that rapid burial is necessary for fossilization.

There are other theories that haven't been conclusively proven but are probably true, like the Big Bang Theory.

Isn't it curious that some theories have wider acceptance than others? Don't you wonder why that is? It is not because the supposed scientific set demands its acceptance.

[Oztrich Boy]

It has been discovered, much to the embarrassment of evolutionists, that the moths are nocturnal and are not found on tree trunks during the day

Noctural means they are ACTIVE during the night and sleep during the day.

"Where they gonna go? Detroit?"

[2 Kool 2 Be 4-Gotten]

Slightly off topic perhaps, but does anyone know anything about the Howard Hughes Medical Institute? My understanding is that you can be a perfesser at Stanford, Yale etc. and if you're a "star" you get asked to be part of the HHMI. I'm sure there are all sorts of economic benefits to the perfesser and his home university and al the rest but what about the internal finances of the HHMI? I just wonder if there are any skeletons rattling around in that closet. Just curious.

[AntiGuv]

Isn't it curious that some theories have wider acceptance than others? Don't you wonder why that is?

I sure don't. Whenever overwhelming evidence conflicts with religious precept a significant number will reject the former in order to retain the latter.

[Eagle of Liberty]

I was astounded when I was in my early 20s to *really learn* about creation as described in Genesis and as further illustrated by creation scientists. Amazing how cohesive it is once you shed your prejudice against it.

I try to never close my mind and will take all views into consideration. You seem to have a lot of info in your Big Brain. I would like it if you could answer a question for me. WHEN were we created? Before dinosaurs, after dinosaurs, we've always been around? I am curious.

[Doctor Stochastic]

Power is the ultimate aphrodisiac. Money is second. Other things show the ability to acquire either of the first two.

[Dataman]

Noctural means they are ACTIVE during the night and sleep during the day.

But they don't reside on bark during sleep.

[Damifino]

A wing is simply a modified foreleg. There is no "wing gene."

Yeah, I was being a bit tongue and cheek with it. The point I was trying to make is; in the million years or so for the foreleg to make its journey to flight, the appendage would be particularly unhelpful to flying or running. I'd think with a million year competitive disadvantage any species would be hard pressed to survive long enough to take wing (or most any other major transition).

[donh]

Extremely bad comparison. Humans are unique in that we lack almost any physical adaptations to the environment. Sauropods had every physical adaption needed to survive as long as they did. The only thing we have is intelligence. Not only is it a useful adaptation, it is our ONLY adaptation.

Not remotely true.

[Junior]

Herein you're wrong. The wing developed on bipedal dinosaurs (therapods). Recent research of certain ground-dwelling birds in their juvenile state shows that even wings that do not permit flight still aid the animal in climbing out of danger. Flapping the undeveloped wings allows the animal to more effectively cling to the surface of the feature (tree, log, rock face) it is climbing to escape predators.

Any advantage in the game of life is likely to propagate throughout the gene pool.

[Junior]

You've been told, by the resident geologists on this forum, no less, that the processes and results of the Mount St. Helens formations are not the same processes and results of, say, the Grand Canyon.

Blank-Slate Syndrome -- a horrible way to go through life.

I thought I'd ping one of those geologists in case you had any questions.

[Dataman]

Whenever overwhelming evidence conflicts with religious precept a significant number will reject the former in order to retain the latter.

Two glaring flaws in that statement:

1) Evolution and creation share the same evidence. Therefore if it is overwhelming evidence, it is also overwhelming for creation. It is the interpretation of that evidence that is in question.

2) Evolution isn't rejected just because it conflicts with the Genesis account. Evolution is rejected because it is bad science, bad logic and not even very good story telling. This is not just the opinion of evolutionists, but a few creationists as well:

We take the side of science in spite of the patent absurdity of some of its constructs, in spite of its failure to fulfill many of its extravagant promises of health and life, and in spite of the tolerance of the scientific community for unsubstantiated just-so-stories, because we have a prior commitment, a commitment to materialism. It is not that the methods and institutions of science somehow compel us to accept a material explanation of the phenomenal world, but, on the contrary, that we are forced by our priori adherence to material causes to create an apparatus of investigation and a set of concepts that produce material explanations, no matter how counterintuitive, no matter how mystifying to the uninitiated. Moreover, that materialism is an absolute, for we cannot allow a Divine Foot in the door." - Richard Lewontin

Just in case--Lewontin is an evolutionist.

[Dataman]

You've been told,

LOL! As if the resident evolutionists have some kind of authority which transcends truth. Nothing I said about layers is incorrect, not even by evolutionary standards.

[dirtboy]

Because one formation of layered sediments formed in a day, that does not therefore mean that all layered sedimentary formations were formed in a short timeframe. And quick burial is required for most terrestial fossils - but many oceanic fossils do not require such - indeed, many limestones are almost entirely composed of coral fossils, such as those forming Guadalupe Peak in Texas (a huge fossil reef complex). And many key indicator fossils - fossils found within a predictable timeframe and across wide parts of the globe - are often hard-shelled microorganisms such as foramnifera that will fossilize whereever they are deposited.

Has geology in the past understated catastrophism as a agent of geologic change? Yep. Does that therefore give a boost to creationism? Nope - just as the shift from a geosynclynic model to a techtonic model for mountain building, something far more profound, did nothing to prove creationism.

[Heartlander]

Try this: Natural selection would have bumped us off if random mutations hadn't been happening extra fast along about then.

According to Lahn, "The making of the large human brain is not just the neurological equivalent of making a large antler. Rather, it required a level of selection that's unprecedented..."

Extra fast and fortuitous random mutations effecting human brain development due to environmental pressure? Hmmm… Lahn says, “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. But Lahn also says, “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…”

Now consider that the last hypothetical common primate ancestor of both humans and the modern ape family lived approx 6 million years ago and had a brain size of 350 cc (approx brain size of a chimp). In regard to the human brain though, its not just size that matters (see antlers) or we would be studying whale and dolphin brain development… or maybe they would be studying us…

Accidental Just In Time Delivery?

Why bring up the USPS?

[Junior]

Okay, let's put it this way: You have been hit over the head with literally mountains of evidence. Of course, you do suffer from Blank-Slate Syndrome, so I guess your inability to retain information can't really be your fault.

[dirtboy]

Nothing I said about layers is incorrect, not even by evolutionary standards.

Yep - that layered formation did form in a few days. However, you then take that one square-peg truth and try to pound it in every stratigraphic round hole that you come across.

[stremba]

You make my point for me. My point has always been that creationism/ID are consistent with ANY observation. Because of that, they are not falsifiable and hence are not scientific theories. If I am wrong, then please give an observation that would be inconsistent with ID/creationism (take your pick). It need not be something that has actually been observed, just something that potentially could be observed. Example, as pertaining to evolution: finding a new species of organism on earth that has different genetic material from all others would falsify common descent, which is a large part of the theory of evolution. Give a similar example for ID or creationism.

[dirtboy]

Oh, and FWIW, I have my own doubts about some of the dynamics of Darwinian evolution. However, the fossil evidence is fairly clear as to the rise, extent and extinction of species over time. How that happens still needs to be fully hashed out. But it did happen, and it has happened over hundreds of millions of years.

[Damifino]

Flapping the undeveloped wings allows the animal to more effectively cling to the surface of the feature (tree, log, rock face)

Yeah, I've noticed how well domesticated chickens, with their poorly developed wings easily run and flap their way to safety.