recapitulation (Homo solus)

[mellowtigger]

I mentioned last year the theory of recapitulation. It explains why a developing embryo goes through different stages that represent the evolutionary history of its ancestors. Most people, for example, are familiar with the gill slits (reminiscent of a fish) that a human embryo briefly grows.

Not explicity stated in the theory, however, is that this historical time warp is not completed for humans by the time of their birth. Human newborns are still partially pre-human in their body configuration. Their historic shape-changing continues well past their first gasps of air. My previous post mentions the chimpanzee trachea, for instance, that does not drop into its human position until a few months after birth.

I have previously claimed (without evidence) that we should expect to see even later changes in the development of the newborn human brain. Since those evolutionary changes that distinguish Homo sapiens from our other ape cousins are the most recent to be added to the human lineage, their chemical signaling should occur very late in human development. They should occur even after the trachea descent.

Now there's evidence to support the idea.

Baby brain growth mirrors changes from apes to humans

"Through comparisons between humans and macaque monkeys, my lab previously showed that many of these high-growth regions are expanded in humans as a result of recent evolutionary changes that made the human brain much larger than that of any other primate," says Van Essen. "The correlation isn't perfect, but it's much too good to put down to chance."

The high-growth regions are areas linked to advanced mental functions such as language, reasoning, and what Van Essen calls "the abilities that make us uniquely human." He speculates that the full physical growth of these regions may be delayed somewhat to allow them to be shaped by early life experiences.

- http://www.physorg.com/news198170497.html

The article summary does not specify the ages which were studied. I expect further research to pinpoint the "final growth" stage of human brain development to occur at exactly that point where many autistic children diverge from the standard population: 30 months of age, give or take a few months.

I think it is no coincidence that parents "lose their child to autism" at age 2-3 years.  Humans still contain the genetic baggage of their ancient ancestors.  Epigenetics keeps the unnecessary bits turned off until needed.  Suppose the final bits that are supposed to turn on to achieve a "human" brain don't get activated?  Suppose a Homo infant keeps its original, older brain configuration?  Even more interesting, suppose it gets something new at age 30 months, something different from what Homo sapiens infants get?

When its time to experiment, cells are able to start turning on bits and pieces of older genetic code to see what produces a useful and successful combination.  Scientists are still working out the chemical signals that encourage (and inhibit) this experimentation.  So far, it sounds like the gender war begins at the cellular level, with male and female parent chemicals warring for control over developing embryonic genetic machinery.

There's already a theory that epigenetic changes precede genetic ones.  As the turn-on/turn-off war continues, different combinations of codes produce new kinds of creatures. They're still the same species as the parent, so far, as new stable combinations are produced. The genes haven't changed yet, only the epigenetic signals.

Combinations that succeed, however, may then develop over time into a new genetic lineage.  Additional gene changes are accumulated to help stabilize the combination, and then it may be fairly represented as a new species. I still claim that autism is exactly one of these changes, one of Mother Nature's attempts to rearrange sentient apes into a form that I call Homo solus.

I eagerly await more research reports.  :)

Comments

[pi3832.livejournal.com]

Does the development process ever stop? Schizophrenia is genetically linked, but typically doesn't manifest itself until adulthood. And we all know that puberty plays holy hell with our brains. When does the human brain reach "maturity"? Is the life cycle really one of maturation, followed by a long, slow decline? Are decay and maturation mutually exclusive?

[mellowtigger]

I distinguish development from maturation.

I would call the development process finished and the resulting creature (plant or animal) "species-specific" whenever body parts have reached the location where they can perform their necessary function. At that point, a creature has all of the tools it needs to survive on its own if it absolutely had to. The last human baby tooth emerges at 33 months. Since that timeframe is also in line with chimpanzee-to-human trachea movement, I think it's a good candidate for when little proto-humans become actual humans.

There is still a lot of maturation that continues, but it adds no new survival feature to the individual animal. Development of body parts is complete. Mostly there is growth-in-size to accomplish, but that process includes two other big changes.

1) As body size increases, there is also a replacement strategy that occurs. Baby teeth are exchanged for adult teeth once the jaw size has increased sufficiently to accommodate the larger versions.

2) As growth-in-size nears completion (providing a body as much competitive advantage as possible) then reproductive function is added via puberty. Puberty is a massive change, but it does not add any functions necessary for the survival of the individual. It's just an energy expenditure that's delayed while the more important growth-in-size continues.

Species with metamorphosis do complicate my little definition.

The brain is an unusual organ because growth-in-size affects its ability to perform its primary function. Necessary interneuron connections can be interrupted by brain cell replication. Maturation-phase (growth in size) and development-phase (addition of functional parts) may be the same thing for the brain.