"Organisms are just a bunch of cells," says John W. Pepper (University of Arizona).
And as everyone knows from high school biology, cells reproduce by splitting and making copies of themselves. When this happens--over and over--mistakes are made, and they produce inaccurate copies. Most of these mistakes--mutations--are not fit for survival and disappear. Others have survival advantages, and continue to replicate. And that's evolution.
So you might ask, if this is what is happening inside our bodies, why don't individual humans evolve over time? Why are you pretty much the same person--except older--than you were ten years ago?
Because, says Pepper, simple cell division is not the way human organisms replace their cells.
Although Occam's Razor advises us that the simplest is often the best, Pepper points to a rather klunky, inelegant process by which human cells replace themselves--and avoid mutation within one person's lifetime.
In order to replace themselves, cells need to both reproduce and proliferate. According to Pepper, undifferentiated stem cells carry the burden of initiating the reproduction of cells by simple division. He hypothesizes that stem cells don't evolve because their populations are largely "small and quiescent." When they reproduce, they initially change just a little, enough to produce new kind of cells--transient amplifying cells (TACs)--which divide, and thereby proliferate, several times, each time becoming more differentiated, until they are mature cells, which can neither reproduce nor replicate.
The new cells, because they can't reproduce, are evolutionary dead ends. "One tissue may include a series of several TAC stages, each of which results from the division of the preceding stage. These cells are transient in the sense that they only proliferate for a limited amount of time before they become terminally differentiated and are eventually shed from the tissue. They amplify the proliferative potential of the somatic stem cells, because each TAC stage doubles the number of descendent cells that ultimately result from the division of a somatic stem cell," writes Pepper in his paper, "Animal Cell Differentiation Patterns Suppress Somatic Evolution," with colleagues Kathleen Sprouffske , Carlo C. Maley (PloS Computational Biology, 2007).
For highly differentiated cells, losing the ability to reproduce was part of the trade-off of becoming part of a multi-cellular organism.
"They got to be part of something more powerful," Pepper said. "Something that was hard to eat and good at eating other things."
So if the multi-stage process of cell reproduction and proliferation suppresses evolution and cell-change, why do we age?
"Somatic evolution has been proposed as a fundamental source of senescence. We have shown that serial differentiation can reduce somatic evolution, but not completely eliminate it. Some proliferation by stem cells is necessary, and self-renewal of TACs can arise by sporadic somatic mutations disrupting normal differentiation. By slowing somatic evolution, serial differentiation may not entirely eliminate senescence but delay it until old age," writes Pepper.
In other words, if it we replaced single cells by simple division, the errors associated with aging might occur quite rapidly. With the muti-stage process--stem cells to TACs--there is a slower accretion of changes characteristic of aging over a longer time.
This research also has implications for cancer research. Cancer is cell-division gone wild.
Simply put by Pepper, "When cells reach the point where they divide constantly, instead of only when needed, they are cancer cells."
He adds, "If you understand the conditions under which they cooperate, you can understand the conditions under which cooperation breaks down. Cancer is a breakdown of cooperation."
And so might be the process of aging.
Pepper JW, Sprouffske K, Maley CC, 2007 Animal Cell Differentiation Patterns Suppress Somatic Evolution. PLoS Comput Biol 3(12): e250. doi: 10.1371/journal.pcbi.0030250
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My book, Nasty, Brutish, and Long: Adventures in Old Age and the World of Eldercare, is being published this week, Thursday, March 19th.
Read my post at the Penguin blog here.
See my News and Events, here. I will likely be on a radio show near you.
