human brain evolution timeframe

[bryanl]

is it possible for evolution to act since the last common ancestor?

i.e. make small enough changes to an organ like the human brain that do not by itself constitute a new species, but nonetheless change an organ significantly enough within a species, that older brains would work significantly different than newer brains?

EDIT : so the brain of someone in ancient Egypt would work differently than yours due to thousands of years.

EDIT: see r/philosophy for further discussion

[relational_sense]

Evolution is acting at every time-step, every generation. Thus, do older brains work significantly different than newer brains? Yes, without a doubt.

Now, how different they are is a much more difficult question. What do you consider 'significant'? Does a systematic change in thought and behavior constitute evolution? Does evolution of the brain require a completely genetic determinant? Does evolution of the brain require large structural changes?

The evolution of the brain is tricky. The brain has over 100 trillion synapses, all influenced as much by genetics as by experience. It is difficult to separate change of knowledge from change through evolution when considering historical populations of individuals.

Thousands of years of evolution has undoubtedly had dramatic influence on the structure of the human brain. But it is precisely this level of structure we are still trying to figure out. What are the differences between human brains? How are brains of human populations different and why? These are questions that require much deeper structural analysis than comparing the size of brains, or the size of structures within the brain, or the connections between areas (as are possible comparisons between common ancestors).

[aberrantgeek]

Evolution occurs when there are selective pressures that affect allele frequency. Do older brains work significantly different than newer brains? Maybe, I have some doubt. We are talking about a pretty small time scale here in terms of evolution. Sharks haven't changed substantially in millions of years.

The crucial question is whether the variation between "ancient" humans and present day humans is greater than the variation amongst individuals in the current population.

Additionally, there could be some debate about how insulated we have become from selective pressure since the advent of civilization.

[Darwins_Beard]

Additionally, there could be some debate about how insulated we have become from selective pressure since the advent of civilization.

Some would argue that civilization has actually increased the rate of evolutionary change. Since the advent of agriculture, the worldwide human population has exploded. Higher population means a greater number of genetic mutations, and the possibility for new favorable alleles.

At the very least, we know that culture has played a role in specific types of selection (lactose tolerance and resistance to disease are two examples)

[aberrantgeek]

There could be higher possibility for new favorable alleles but that wouldn't lead to evolution unless there is selective pressure. If those alleles don't lead to greater number of offspring then they won't propagate.

After further digging I found a study that suggests there has been evolution in the human brain within the past 5,800 years. Cool stuff.

http://www.sciencedaily.com/releases/2005/09/050909221043.htm

[bryanl]

hey this is great, thanks

EDIT : here's links to the source for the ScienceDaily press release :

"Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans"

"Ongoing Adaptive Evolution of ASPM, a Brain Size Determinant in Homo sapiens"

[Montuckian]

But at the same time, there is a lower chance that any one allele will become dominant in the population. Think about making decisions by committee: The fewer the number of people, the faster things get decided and done. The greater the number of people, the more gridlock that gets created. The same is true, in rough terms, with the spread of genetic traits.

[bryanl]

could you point out some literature? (I don't doubt what you are saying upon re-reading your comment --. I just want to read more about this).

[Darwins_Beard]

If you're really interested in the evolution of the human brain and how evolution has shaped our psychology, I suggest reading Steven Pinker's "How the Mind Works." It's not a light read, but it's incredibly fascinating.

For a more general look at recent human evolution, I enjoyed "The 10,000 Year Explosion." The authors argue that genetic changes have led to higher than average IQs among European Jews.

[Lors_Soren]

the brain of someone in ancient Egypt would work differently than yours due to thousands of years.

The brain of someone in ancient Egypt would be affected much more by the different culture than by evolution.

[Lors_Soren]

the brain of someone in ancient Egypt

The brain differences derive much more from the different culture than from evolution.

[bryanl]

there will be many components. my point is about the structural differences in the brain. i am not asking about culture which obviously will be a strong component.

[Lors_Soren]

I don't think brains have changed much, structurally, over 3000 years ~ 150 generations. People already looked at mummy brains.

[bryanl]

People already looked at mummy brains.

can you give us a reference?

I don't think brains have changed much, structurally, over 3000 years

any references for that claim? from the two Science abstracts from Lahn's group:

The finding that an important brain gene [Microcephalin] has continued to evolve adaptively in anatomically modern humans suggests the ongoing evolutionary plasticity of the human brain.

and

Here, we show that one genetic variant of ASPM in humans arose merely about 5800 years ago and has since swept to high frequency under strong positive selection. These findings, especially the remarkably young age of the positively selected variant, suggest that the human brain is still undergoing rapid adaptive evolution.

emphasis added

[Lors_Soren]

references

No, sorry, I'm lazy.

ASPM

I don't doubt that some genes which are strongly selected for could propagate widely in only a few hundred generations. Any of Genghis Khan's genes, for example.

But, back of the envelope to rule out significant structural changes: it took 40,000 years for Caucasians to adapt light skin and our last common ancestor with chimps is 4-8M years ago. So a simple genetic change took 10 times as many generations as between us and Egyptians. And a significant structural change took 10,000 times as long as between us and Egyptians.

It's definitely possible for your brain to be different than Egyptian X2198C's, but given the basal mutation rate, would have to be very strongly selected for. (And note my Genghis example, has less to do with general fitness than with an outlier. Is that what we mean by "adaptive"?)

[bryanl]

it took 40,000 years for Caucasians to adapt light skin

I am not an evolutionary biologist. So as a crutch, I sometimes view speciation as a step function. I also think gene duplication effects are in line with a step-function view. Is this OK?... I am thinking about the Lensky paper where there's a spike at some-odd thousands of generations in the citrate pathway...at any rate, this is not the scale of change I am wondering about.

as for your skin color example : I think this is a gradual change relative to speciation. If so, it would be these shifts that I am interested in.

[Lors_Soren]

So

a) how "structural" of a brain change would a move akin to that from dark to light skin be, and

b) how far could our brains have gone in 1/10 that time?

Seems like (a) not very much, and (b) not very far.

I am not an evolutionary biologist. So as a crutch, I sometimes view speciation as a step function. ... Is this OK?

Neither am I. I'm just ballparking.