Origins and Evolution
P.Z. Meyers at Pharyngula has two great biology posts today, in addition to the political humor cited below.
First, Cesarean-Section births may be allowing humans to grow larger brains.
Babies have very big heads that squeeze with only great difficulty through a relatively narrow pelvis, so the relationship in size between head diameter and the diameter of the pelvic opening has been a limitation on human evolution. We know this had to be a factor in our evolution: the average newborn mammal has a cranial capacity that is roughly 50% of the adult size, chimpanzee babies have heads about 40% of the adult size, but human babies have crania that are only 23% of what they will be in adults.
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This is the subject of an article by Joseph Walsh in the American Biology Teacher, which suggests that C-sections will have an effect on human evolution.
“Nothing in biology makes sense except in the light of evolution.” This was the title of an essay by geneticist Theodosius Dobzhansky writing in 1973. Many causes have been given for the increased Cesarean section rate in developed countries, but biologic evolution has not been one of them. The C-section rate will continue to rise, because the ability to perform a safe C-section has liberated human childbirth from natural selection directed against too small a maternal pelvis and too large a fetal head. Babies will get bigger and pelves will get smaller because there is nothing to prevent it.
That increasing availability of C-sections might lead to an evolutionary shift towards increasing cranial capacity at birth is a reasonable hypothesis, but I’m not convinced that it has been convincingly demonstrated yet. There are too many variables that effect brain size at birth to make a clean analysis possible; in addition, many of the measures are indirect. Often, we use birth weight as a proxy for cranial capacity, and that means the numbers and correlations are sloppier than they should be. Many of the measurements made are of factors that are readily influenced by the environment, which makes it difficult to imply that these are the product of genetics.
So the idea is weakly supported, but tantalizing. Even as a purely theoretical exercise, though, what it does say is that it is obvious that human culture cannot end human evolution…all it can do is shape the direction in which it can occur.
[Emphasis mine.]
That’s the way science works: even if, especially if, you like an idea, you must remain skeptical, and try your best to disprove it, to falsify it. Science isn’t a collection of eternal, unquestioned truths — it’s a protocol for throwing out the trash. Scientists are janitors, not priests.
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Scientists are also custodians, in the sense that they care for things.
P.Z. Myers also points out that Old Scientists never clean out their refrigerators; they just keep meticulous notes about the contents.
We all know the story of the Miller-Urey experiment. In 1953, a young graduate student named Stanley Miller ran an off-the-wall experiment: he ran water, methane, ammonia, and hydrogen in a sealed flask with a pair of electrodes to produce a spark, and from those simple building blocks discovered that more complex compounds, such as amino acids, were spontaneously produced. Stanley Miller died in 2007, and in going through his effects, the original apparatus was discovered, and in addition, several small sealed vials containing the sludge produced in the original experiment were also found.
This isn’t too surprising. I’ve gone through a few old scientists’ labs, and you’d be surprised at all the antiquities they preserved, all with notes documenting exactly what they are. It’s habit to keep this stuff.
Now the cool part, though: the scientists who unearthed the old samples ran them through modern analysis techniques, which are a bit more sensitive than the tools they had in the 1950s. In 1953, Miller reported the recovery of five amino acids from his experiment. The reanalysis found twenty two amino acids and five amines in the vials. He was more successful than he knew!
Tags: Evolution, Miller-Urey, P.Z. Meyers, Pharygula, scientific method