brain evolution

Re-organization more important than changes in brain size

A new finding points to brain reorganization, rather than brain size, as the driver in primate brain evolution. Data from 17 anthropoid primate species (including humans) across 40 million years has found that around three quarters of differences between the brains of species of monkeys and apes are due to internal reorganization that is independent of size. The

04/2013

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Crucial factors in the evolution of the human brain

September, 2012

Two recent studies comparing gene expression in the brains of human and other animals reveal a key protein for brain size and others for connectivity and regulation.

Genetic comparisons have pinpointed a specific protein as crucial for brain size, both between and within species. Another shows how genetic regulation in the frontal lobes distinguishes the human brain from that of closely related species, and points to two genes in particular as critical.

The protein determining brain size

Comparison of genome sequences from humans and other animals has revealed what may be a crucial protein in the development of the human brain. The analysis found that humans have more than 270 copies of a protein called DUF1220 — more than any other animal studied — and that the number of copies in a species seems to match how close they are to us. Chimpanzees, for example, have 125, and gorillas 99, while marmosets have only 30, and mice just one.

Moreover, comparison of humans with microcephaly and macrocephaly reveals that those with microcephaly (“small brain”) have lower numbers of this protein than normal for humans, and those with macrocephaly (“large brain”) have higher numbers. Copy numbers of the protein were also correlated with gray matter volume in humans without these brain disorders.

In other words, evidence from three lines of inquiry converge on DUF1220 copy number being associated with brain size.

Differences in gene expression and connectivity

But the development of the human brain is not only about size. The human brain is more complex, more connected, than the brains of most other animals. Another genetic analysis has been comparing gene activity in humans, chimpanzees and rhesus macaques, using post-mortem brain tissue of three regions in particular – the frontal cortex, hippocampus and striatum.

Gene expression in the frontal lobe of humans showed a striking increase in molecular complexity, with much more elaborate regulation and connection. The biggest differences occurred in the expression of human genes involved in plasticity.

One gene in particular stood out as behaving differently in the human brain. This gene — called CLOCK, for obvious reasons — is thought to be the master regulator of our body’s clocks. The finding suggests it has influence beyond this role. Interestingly, this gene is often disrupted in mood disorders such as depression and bipolar syndrome.

A second important distinction was how many more connections there were in human brains among networks that included the language genes FOXP1 and FOXP2.

In comparison to all this, gene expression in the caudate nucleus was very similar across all three species.

The findings point to the role of learning (the genes involved in plasticity) and language in driving human brain evolution. They also highlight the need to find out more about the CLOCK gene.

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Memory loss in old age the price we pay for a large brain & a long life?

September, 2011
  • Chimpanzee brains don’t shrink with age as humans’ do. It may be that cognitive impairment and even dementia are our lot because we work our brains too hard for too long.

Comparison of 99 chimpanzee brains ranging from 10-51 years of age with 87 human brains ranging from 22-88 years of age has revealed that, unlike the humans, chimpanzee brains showed no sign of shrinkage with age. But the answer may be simple: we live much longer. In the wild, chimps rarely live past 45, and although human brains start shrinking as early as 25 (as soon as they reach maturity, basically!), it doesn’t become significant until around 50.

The answer suggests one reason why humans are uniquely vulnerable to Alzheimer’s disease — it’s all down to our combination of large brain and long life. There are other animals that experience some cognitive impairment and brain atrophy as they age, but nothing as extreme as that found in humans (a 10-15% decline in volume over the life-span). (Elephants and whales have the same two attributes as humans — large brains and long lives — but we lack information on how their brains change with age.)

The problem may lie in the fact that our brains use so much more energy than chimps’ (being more than three times larger than theirs) and thus produce a great deal more damaging oxidation. Over a longer life-span, this accumulates until it significantly damages the brain.

If that’s true, it reinforces the value of a diet high in antioxidants.

Reference: 

[2500] Sherwood, C. C., Gordon A. D., Allen J. S., Phillips K. A., Erwin J. M., Hof P. R., et al.
(2011).  Aging of the cerebral cortex differs between humans and chimpanzees.
Proceedings of the National Academy of Sciences. 108(32), 13029 - 13034.

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Large brains in mammals first evolved for better sense of smell

July, 2011

High-tech X-ray scans of ancient fossil skulls have revealed that the increase in brain size that began with the first mammals was driven by improvements in smell and touch.

190-million-year-old fossil skulls of Morganucodon and Hadrocodium, two of the earliest known mammal species, has revealed that even at this early stage of mammalian evolution, mammals had larger brains than would be expected for their body size. High-resolution CT scans of the skulls have now shown that this increase in brain size can be attributed to an increase in those regions dealing with smell and touch (mammals have a uniquely well developed ability to sense touch through their fur).

Comparison of these fossils with seven fossils of early reptiles (close relatives of the first mammals), 27 other primitive mammals, and 270 living mammals, has further revealed that the size of the mammalian brain evolved in three major stages. First, an initial increase in the olfactory bulb and related areas (including the cerebellum) by 190 million years ago; then another jump in the size of these regions shortly after that time; and finally an increase in those regions that control neuromuscular coordination by integrating different senses by 65 million years ago.

It’s speculated that the initial increase in smell and touch was driven by early mammals being nocturnal — dinosaurs being active during the day.

Reference: 

[2301] Rowe, T. B., Macrini T. E., & Luo Z-X.
(2011).  Fossil Evidence on Origin of the Mammalian Brain.
Science. 332(6032), 955 - 957.

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Old bees' memory fades too

November, 2010
  • New research shows that many old bees, like many older humans, have trouble replacing out-of-date knowledge with new memories.

I love cognitive studies on bees. The whole notion that those teeny-tiny brains are capable of the navigation and communication feats bees demonstrate is so wonderful. Now a new study finds that, just like us, aging bees find it hard to remember the location of a new home.

The study builds on early lab research that demonstrated that old bees find it harder to learn floral odors. In this new study, researchers trained bees to a new nest box while their former nest was closed off. Groups composed of mature and old bees were given several days in which to learn the new home location and to extinguish the bees' memory of their unusable former nest box. The new home was then disassembled, and groups of mixed-age bees were given three alternative nest locations to choose from (including the former nest box). Some old bees (those with symptoms of senescence) preferentially went to the former nest site, despite the experience that should have told them that it was unusable.

The findings demonstrate that memory problems and increasing inflexibility with age are not problems confined to mammals.

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Sign language study shows multiple brain regions wired for language

April, 2010

Perhaps we should start thinking of language less as some specialized process and more as a particular approach to thought. A study involving native signers of American Sign Language adds to the increasing body of evidence that we process words in the same way as we do the concepts represented by the words; speaking (or reading) is, neutrally speaking, the same as doing.

Perhaps we should start thinking of language less as some specialized process and more as one approach to thought. A study involving native signers of American Sign Language (which has the helpful characteristic that subject-object relationships can be expressed in either of the two ways languages usually use: word order or inflection) has revealed that there are distinct regions of the brain that are used to process the two types of sentences: those in which word order determined the relationships between the sentence elements, and those in which inflection was providing the information. These brain regions are the ones designed to accomplish tasks that relate to the type of sentence they are trying to interpret. Word order sentences activated areas involved in working memory and lexical access, including the dorsolateral prefrontal cortex, the inferior frontal gyrus, the inferior parietal lobe, and the middle temporal gyrus. Inflectional sentences activated areas involved in building and analyzing combinatorial structure, including bilateral inferior frontal and anterior temporal regions as well as the basal ganglia and medial temporal/limbic areas. In other words, as an increasing body of evidence tells us, we process words in the same way as we do the concepts represented by the words; speaking (or reading) is, neutrally speaking, the same as doing.

Reference: 

[453] Newman, A. J., Supalla T., Hauser P., Newport E. L., & Bavelier D.
(2010).  Dissociating neural subsystems for grammar by contrasting word order and inflection.
Proceedings of the National Academy of Sciences. 107(16), 7539 - 7544.

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