FINNISH GRANDMOTHER: Research in detailed Finnish birth, marriage and death records is revealing why human women survive past fertility: they help their grandchildren survive. Image: COURTESY OF VIRPI LUMMAA
No animal compares to humans when it comes to studying populations over time. Easy to track and occasionally living in relative isolation, Homo sapiens is the only species that keeps detailed records. That is why biologist Virpi Lummaa of the University of Sheffield in England started in 1998 to comb through Finnish church records from two centuries ago for clues about the influence of evolution on reproduction.
"I always wanted to work on primates," Lummaa says. "But if I wanted to collect a similar data set on wild chimps, I would be struggling. I've decided to study another primate in the end."
The 33-year-old Finnish biologist, aided by genealogists, has pored through centuries-old tomes (and microfiche) for birth, marriage and death records, which ended up providing glimpses of evolution at work in humanity's recent ancestors. Among them: that male twins disrupt the mating potential of their female siblings by prenatally rendering them more masculine; mothers of sons die sooner than those of daughters, because rearing the former takes a greater toll; and grandmothers are important to the survival of grandchildren. "I'm trying to understand human reproductive behavior from an evolutionary perspective," Lummaa says.
Most recently, Lummaa and her colleagues studied the effect of males on their female twins. Of 754 twins born between 1734 and 1888 in five towns in rural Finland, girls from mixed-gender pairs proved 25 percent less likely to have children, had at least two fewer children, and were more than 15 percent less likely to marry than those born with a sister. This impact remained the same regardless of social class and other cultural factors and even if the male twin died within three months of birth, leaving the female twin to be reared as if she was an only child, the researchers reported in Proceedings of the National Academy of Sciences USA.
Lummaa speculates that the findings may be prompted by the male hormone testosterone crossing the womb and masculinizing the female twin, as has been seen in animal studies. Whatever the cause, there's no question of the outcome: Data shows that mothers of opposite-sex twins end up with 19 percent fewer grandchildren than moms of same-sex twins, meaning evolution would seem to favor the latter.
The results are somewhat puzzling, says Ken Weiss, biological anthropologist and geneticist at Pennsylvania State University, noting that "if twinning is genetic, then there should be a slight selection bias against it, so that twinning would be kept rare. But some animals twin routinely." Given the seeming conflict, he says, "there are dangers in over-interpreting 'fitness' effects, even if the observation is correct."
Modern medicine and nutrition tend to obscure these kinds of results as well, hence the need to go back to the preindustrial Finns, before the advent of birth control and the easing of periodic famine and high child mortality rates. "It's almost a shock when you realize that 100 to 150 years ago, 40 percent of babies died before they reached adulthood," even when adulthood was defined as age 15, Lummaa notes.
"In the absence of cultural practices such as contraceptives and assisted reproduction, humans are subject to the same evolutionary forces as are other organisms," says biologist Tobias Uller of the University of Wollongong in Australia. "Given that Virpi's data is extraordinarily detailed compared to what we have available for most other animals, the human data can profitably be used to address key issues in evolutionary theory."
The evolutionary biologist has also used this historical data set to ponder the conundrum of grandmothers. That is, why human women often live long after they are able to reproduce (on average around the age of 50), unlike almost all other animals. "If your ultimate purpose in life was to create as many offspring as possible or pass off as many genes," Lummaa says, "it's kind of strange that human women stop halfway."