Another one was when I found out that we not only have vestigial organs, we have vestigial behaviors. You probably know a vestigial organ is sort of an evolutionary leftover, something our ancestral species had some use for -- but the usefulness went away, leaving behind a functionless structure (a familiar example is the human appendix, which apparently evolved in the context of breaking down cellulose in animals with a high-plant-material diet). I'd known about this since Ms. Jane Miller's high school biology class. But I didn't find out until college that we have some responses that are vestigial -- like goosebumps.
Goosebumps occur because of the little arrector pili muscles that surround each hair follicle, mostly in our arms, legs, and the back of the neck. When these contract, it makes the hair stand on end, leading to the familiar experience of "goosebumps" or "gooseflesh." (I'm not sure why geese have apparently cornered the market on this phenomenon.) So the structure itself is vestigial, but it turns out, so is our response to what triggers it. We typically get goosebumps when we're cold or when we're frightened, alarmed, or threatened. But why?
The answer lies in observing cats. If you're a cat owner, you've probably seen your pet with its hair standing on end, usually in one of two situations -- when it's cold (so that its raised hair traps heat better), or when it's angry (so that it looks bigger and more threatening). Very few humans have enough body hair that they are significantly warmer or more threatening when they have goosebumps. Fortunately. But we still retain the response, even though it's not much use any more.
Old evolutionary habits are hard to break, apparently.
Well, as with most things, it turns out that it's not that simple, and is even cooler. A paper this week in the journal Cell, written by a team led by Yulia Shwartz of the Department of Stem Cell and Regenerative Biology at Harvard University, describes research that shows that the underlying neurology of goosebumps might serve a purpose after all -- stimulating hair growth. As the nerves fire that trigger goosebumps, neurotransmitters like norepinephrine are secreted into the tissue surrounding the hair follicles, and these act as a chemical signal that modulate gene activity in some pretty fundamental stretches of DNA, including the critical developmental gene Sonic Hedgehog.
The way the whole thing sets up is fascinating. "We discovered that the signal comes from the developing hair follicle itself," Shwartz said, in an interview with Science Daily. "It secretes a protein that regulates the formation of the smooth muscle, which then attracts the sympathetic nerve. Then in the adult, the interaction turns around, with the nerve and muscle together regulating the hair follicle stem cells to regenerate the new hair follicle. It's closing the whole circle -- the developing hair follicle is establishing its own niche."
The function of the system works by a feedback mechanism, Shwartz said. "This particular reaction is helpful for coupling tissue regeneration with changes in the outside world, such as temperature. It's a two-layer response: goosebumps are a quick way to provide some sort of relief in the short term. But when the cold lasts, this becomes a nice mechanism for the stem cells to know it's maybe time to regenerate new hair coat."
And again, we retain this response even though most of us aren't that hairy any more. The underlying neurological structure hasn't gone away.
Makes me wonder what other evolutionary leftovers we have still hanging around in our behaviors and responses. We are constructs of our lineage, for better or worse, inheriting both its advantages and its constraints. As advanced as we like to think we are, maybe a lot of what we do is because of some quirk of our ancestry -- which is both fascinating and humbling.
This week's Skeptophilia book recommendation of the week is about as cutting-edge as you can get, and is as scary as it is fascinating. A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution, by Jennifer Doudna and Samuel Sternberg, is a crash course in the new genetic technology called CRISPR-Cas9 -- the gene-editing protocol that Doudna herself discovered. This technique allows increasingly precise cut-and-paste of DNA, offering promise in not just treating, but curing, deadly genetic diseases like cystic fibrosis and Huntington's disease.
But as with most new discoveries, it is not without its ethical impact. The cautious are already warning us about "playing God," manipulating our genes not to eliminate disease, but to enhance intelligence or strength, to change personal appearance -- or personality.
A Crack in Creation is an unflinching look at the new science of gene editing, and tries to tease out the how much of what we're hearing is unwarranted fear-talk, and how much represents a genuine ethical minefield. Doudna and Sternberg give the reader a clear understanding of what CRISPR-Cas9 is likely to be able to do, and what it won't, and maps out a direction for the discussion to take based on actual science -- neither panic and alarmism, nor a Panglossian optimism that everything will sort itself out. It's a wonderful introduction to a topic that is sure to be much in the news over the next few years.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]