“Darwinian Medicine, a relatively new discipline, stems from the conviction that understanding evolutionary history can enhance knowledge of human medical conditions,” said Prof. Robert Martin. “Comparative information, not only from other primates but from other vertebrates generally, yields general principles that, in combination with epidemiological evidence, can throw new light on the origins of human disorders.”
Prof. Robert Martin is Emeritus Curator of Biological Anthropology at the Field Museum, Chicago, Academic Guest of the Institute of Evolutionary Medicine at the University of Zürich, and current STIAS fellow. He presented his work at the fifth STIAS public lecture of 2016.
He discussed selected examples from both female and male aspects of reproduction which illustrate how an evolutionary perspective can yield new insights.
“My main interest is the evolution of primates,” he said. “But we need a broad understanding of primate evolution to understand human evolution.”
One of the research questions Prof. Martin addressed is why so many sperms are needed per ejaculation to fertilise just one egg.
“The generally accepted viewpoint has been that many sperms are needed because they compete to be the fastest and reach the egg first.” said. Prof. Martin. “The evidence tells me that it is more complex.”
“Studies have shown that sperms are not good swimmers and that it is not necessarily the first one to reach the egg that fertilises it.”
Prof. Martin explained that the size of the male testes and therefore the production of sperms has evolved to match social interactions.
“Compared with other primates, human males have smaller testes relative to body size. Because we have evolved to live in single-male relationships (either monogamous or polygamous, with more than one female), there is no need to compete directly with other males. So increased sperm production is unnecessary.”
“If you compare humans to chimps, for example, this becomes much clearer,” continued Martin. “Relative to their body size, chimps have much larger testes than humans – but they live in multi-male troops in which several adult males compete directly to fertilise the females.”
“Other researchers have claimed that some sperms have different functions – serving as hunter-killers, blockers or egg-getters. But, in fact, around 30% of human sperms are visibly deformed. The proportion of abnormal sperms is much lower in chimps – less than 5% – because most defective sperms are eliminated due to intense competition between males.”
Game changers
Prof. Martin has also examined variation in pregnancy lengths in different mammals, leading to the revolutionary interpretation that mounting evidence indicates that sperms are stored in the female reproductive tract in higher primates (monkeys, apes and humans), such that fertilisation may not always take place soon after coitus. “There is evidence that sperms can survive for up to 14 days in a woman’s womb and also that the stored sperms are just as viable as fresh sperms. These sperms are stored in the crypts of the cervix and slowly released to make their way to the oviducts,” said Martin. ‘We know this happens in other species – some bats, for example, can store sperms for about three months. This means that time of coitus is not a reliable starting-point for measuring pregnancy length. It may also mean that our understanding of the so-called ‘fertile period’ in a woman’s cycle is woefully misleading.”
“This is a game-changing interpretation,” said Martin, “because it may mean that there is no well-defined mid-cycle period of fertility.”
Prof. Martin then went on to discuss pregnancy, starting with morning sickness and its association with protecting the fetus. “It seems clear that morning sickness – which is sparked by high levels of the hormone human chorionic gonadotropin (hCG) – seems to protect against problems in fetal development. Miscarriage and fetal death are less frequent with maternal nausea and even less with vomiting.”
He also tackled the ‘immunological paradox’ – the age-old question of why a woman does not reject a fetus as a foreign body. “I think there are now very clear examples from other mammals that indicate the problem of fetal rejection has been solved by recruiting genes from retroviruses that have evolved to circumvent the host’s immunological defences,” said Prof. Martin.
“Retroviruses are very insidious,” continued Martin. “The envelope proteins help to bypass the host’s immune system and retroviruses insert their genome into the host genome. Obviously this is very dangerous with retroviruses like HIV.”
“But, from an evolutionary viewpoint, retroviruses have made their way into the placental genes – particularly in mammals like humans who have highly invasive placentas – and this has developed a protective effect, stopping the mother from rejecting the fetus.”
In the course of his presentation, Prof. Martin mentioned adverse effects of endocrine disrupters in human reproduction, both with respect to a widespread decline in sperm counts and in relation to fertilisation and fetal development. He focused specifically on BPA – bisphenol A – an industrial chemical that has been extensively used to make hard polycarbonate plastics and for many other purposes since the 1960s. BPA is, for instance, used to line food cans and water pipes and even for the printing of many till receipts. He believes that this chemical, which behaves like an oestrogen, has contributed to the increase in reproductive disorders and cancers in both men and women. The topic of endocrine disrupters is a focus of his research project at STIAS.
“There have been marked declines in human sperm counts since the 1930s – happening at different times in different countries – although, unfortunately, thus far there are no data from developing countries,” said Martin. “There is also an increase in male reproductive problems like undescended testes (cryptorchidism) and testicular cancer. Interestingly, this is also happening in dogs, which live particularly close to humans.”
“Research conducted as far back as the 1930s showed that endocrine disrupters such as BPA act like oestrogens and spark activity in the reproductive tract in female mice whose ovaries have been removed. There are also studies of mice which link BPA in pregnancy to obesity and early puberty in the offspring.”
“I see BPA as the modern equivalent of DDT,” said Martin. “As a chemical known to act like an oestrogen, it should never have been used on an industrial scale and it is a disgrace that so far it has been banned only in France.”
Prof. Martin writes a regular blog for Psychology Today – see www.psychologytoday.com/blog/how-we-do-it
Michelle Galloway: Part-time media officer at STIAS
Photograph: Christoff Pauw