Sophie Bushwick (Twitter, Tumblr) is a science journalist and podcaster, and is currently an intern at DISCOVERmagazine.com. She has written for Scientific American, io9, and DISCOVER, and has produced podcasts for 60-Second Science and Physics Central.
Human chromosomes (grey) capped by telomeres (white)
U.S. Department of Energy Human Genome Program
Renowned biologist Elizabeth Blackburn has said that when she was a young post-doc, “Telomeres just grabbed me and kept leading me on.” And lead her on they did—all the way to the Nobel Prize in Medicine in 2009. Telomeres are DNA sequences that continue to fascinate researchers and the public, partially because people with longer telomeres tend to live longer. So the recent finding that older men father offspring with unusually lengthy telomeres sounds like great news. Men of advanced age will give their children the gift of longer lives—right? But as is so often the case in biology, things aren’t that simple, and having an old father may not be an easy route to a long and healthy life.
Every time a piece of DNA gets copied, it can end up with errors in its sequence, or mutations. One of the most frequent changes is losing scraps of information from each end of the strand. Luckily, these strands are capped with telomeres, repeating sequences that do not code for any proteins and serve only to protect the rest of the DNA. Each time the DNA makes a copy, its telomeres get shorter, until these protective ends wear away to nothing. Without telomeres, the DNA cannot make any more copies, and the cell containing it will die.
But sperm are not subject to this telomere-shortening effect. In fact, the telomeres in sperm-producing stem cells not only resist degrading, they actually grow. This may be thanks to a high concentration of the telomere-repairing enzyme telomerase in the testicles; researchers are still uncertain. All they know is that the older the man, the longer the telomeres in his sperm will be.
Every few years it seems that the British biologist Steve Jones declares the death of evolution by natural selection in the human species. The logic here is simple even to a schoolboy: evolution requires variation in fitness, and with declining risk by death during our reproductive years humans have abolished the power of selection. But this confuses the symptom for the disease. Death is simply one way that natural selection can occur. Michelle Duggar has 19 children. The average American woman has around two by the end of her reproductive years. It doesn’t take a math whiz to figure out that Michelle Duggar is more “fit” in the evolutionary sense than the average bear. Even without high rates of death, some people have more children than other people, and if those people who have more children than those who do not are different from each other in inherited traits, evolution must occur. Q.E.D.
But you probably shouldn’t be convinced by logic alone. Science requires theory, experiment, and observation. (If you’re talking humans, you can remove the second from the list of possibilities: there are certain unavoidable ethical obstacles to experimenting on human evolution—plus we take far too long to reproduce.) But humans sometimes have something which bacteria can not boast: pedigrees! Not all humans, of course. Like most of the world’s population I don’t have much of a pedigree beyond my great-grandparents’ generation. But luckily for biologists, the Catholic Church has long taken a great interest in life events such as baptism, marriage, and death, and recorded this info parish by parish. With these basic variables, demographers can infer the the rough life histories of many local populations over the centuries. In many European nations, these databases can go more than 10 generations back. And some aspects of human evolution are revealed by these records.
What aspects am I talking about? Reproduction itself. Not only is variation in fitness one of the primary ways by which evolution occurs, but it is also a trait upon which evolution operates! How else are there rabbits which breed like…rabbits, and pandas…which don’t. There is often variation within species for the odds of multiple births, age at first reproduction, and lifespan, depending upon the differences in selection pressures over a population. And that seems to be exactly what occurs in human beings. There is interesting evidence for evolution of reproductive patterns from populations as diverse African pygmies and Finns, but more recently some researchers have been plumbing the depths of the records of the Roman Catholic Church in Quebec, and they’ve come back with gold.