Human brain result of 'extraordinarily fast' evolution

The sophistication of the human brain is not simply the result of steady evolution, according to new research. Instead, humans are truly privileged animals with brains that have developed in a type of extraordinarily fast evolution that is unique to the species.

"Simply put, evolution has been working very hard to produce us humans," said Bruce Lahn, an assistant professor of human genetics at the University of Chicago and an investigator at the Howard Hughes Medical Institute.

"Our study offers the first genetic evidence that humans occupy a unique position in the tree of life."

Professor Lahn's research, published this week in the journal Cell, suggests that humans evolved their cognitive abilities not owing to a few sporadic and accidental genetic mutations - as is the usual way with traits in living things - but rather from an enormous number of mutations in a short period of time, acquired though an intense selection process favouring complex cognitive abilities.

Evolutionary biologists generally argue that humans have evolved in much the same way as all other life on Earth. Mutations in genes from one generation to the next sometimes give rise to new adaptations to a creature's environment.

Those best adapted to their environment are more likely to survive and pass on their genes to the next generation.

The evolution of a large brain in humans, then, can be seen as similar to the process that leads to longer tusks or bigger antlers. In general terms, and after scaling for body size, brains get bigger and more complex as animals get bigger.

But with humans, the relative size of the brain does not fit the trend - our brains are disproportionately big, much bigger even than the brains of other non-human primates, including our closest relatives, chimpanzees.

Prof Lahn's team examined the DNA of 214 genes involved in brain development in humans, macaques, rats and mice.

By comparing mutations that had no effect on the function of the genes with those mutations that did, they came up with a measure of the pressure of natural selection on those genes.

The scientists found that the human brain's genes had gone through an intense amount of evolution in a short amount of time - a process that far outstripped the evolution of the genes of other animals.

"We've proven that there is a big distinction," Prof Lahn said. "Human evolution is, in fact, a privileged process because it involves a large number of mutations in a large number of genes.

"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."

As for how all of this happened, the professor suggests that the development of human society may be the reason.

In an increasingly social environment, greater cognitive abilities probably became more of an advantage.

"As humans become more social, differences in intelligence will translate into much greater differences in fitness, because you can manipulate your social structure to your advantage," he said.

"Even devoid of the social context, as humans become more intelligent, it might create a situation where being a little smarter matters a lot.

"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."

In the large scale, the fossil record gets sparser as you go back, yes. For instance, it's very hard to find Precambrian sediments that haven't been squished to where they're more metamorphic than sedimentary and thus don't have any fossils.

Here's the striking thing about what the fossil record shows about radiations like early carnivores, the great-granddaddy of them all probably being the Miacids. Pick any modern line: dog, cat, bear, weasels, civets, etc. Trace it back in time through the fossils. The farther back you go, the less a specimen looks like the modern form and the more it looks like its contemporaries on the other lines. That is, you can see the divergence in reverse.

For that matter, when you get back to those miacids, they don't look that different from their contemporaries, the ancestors of the hoofed mammals (ungulates).

This paper explains fossil record and taxonomy issues wonderfully: Taxonomy, Transitional Forms, and the Fossil Record.

Moving further up the taxonomic hierarchy, the condylarths and primitive carnivores (creodonts, miacids) are very similar to each other in morphology (Fig. 9, 10), and some taxa have had their assignments to these orders changed. The Miacids in turn are very similar to the earliest representatives of the Families Canidae (dogs) and Mustelidae (weasels), both of Superfamily Arctoidea, and the Family Viverridae (civets) of the Superfamily Aeluroidea. As Romer (1966) states in Vertebrate Paleontology (p. 232), "Were we living at the beginning of the Oligocene, we should probably consider all these small carnivores as members of a single family." This statement also illustrates the point that the erection of a higher taxon is done in retrospect, after sufficient divergence has occurred to give particular traits significance.

Figure 10. Comparison of skulls of the early ungulates (condylarths) and carnivores. (A) The condylarth Phenacodus possessed large canines as well as cheek teeth partially adapted for herbivory. (B) The carnivore-like condylarth Mesonyx. The early Eocene creodonts (C) Oxyaena and (D) Sinopa were primitive carnivores apparently unrelated to any modern forms. (E) The Eocene Vulpavus is a representative of the miacids which probably was ancestral to all living carnivore groups. (From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)

Here's a bit of info, not all that much, on Nature's peer review process: publication policies. Click on number 8, "Referee policy." Also, in the left margin, click on "How to get published in Nature." Basically, the editor picks the peers. It says:

Most papers are sent to two or three referees, but some are sent to more or, occasionally, just to one. Referees are chosen for the following reasons:
* independence from the authors and their institutions
* ability to evaluate the technical aspects of the paper fully and fairly
* currently or recently assessing related submissions
* availability to assess the manuscript within the requested time.

I think that every publication has its own rules, but the concept of peer review is pretty much the same everywhere.

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