Did 'burrowing' placenta give us big brains?
- Did 'burrowing' placenta give us big brains?* 08 July 2008* From New Scientist Print Edition. Subscribe and get 4 free issues.* Andy CoghlanONE of the biggest puzzles of evolution is how our brains got to be big compared with those of other animals. It now seems possible that a hormone which allows the placenta to burrow into the wall of the uterus in some primates might also have played a key role in brain evolution.“A hormone which allows the placenta to burrow into the uterus wall might have played a key role in brain evolution”Our brains account for a whopping 2.4 per cent of our body weight, three times the corresponding percentage in one of our nearest relatives, the orang-utan, and 35 times that of the lemur. Another, seemingly unrelated, curiosity separates humans and primates from other mammals. In humans the placenta burrows one-third of the way into the smooth, muscled wall of the uterus, while in other primates the placenta is less invasive - and in most mammals the placenta does no burrowing at all.Researchers had already speculated that the burrowing placenta gave the fetus access to extra maternal blood, providing vital nutrients that allowed large brains to evolve. There is some evidence for this link: in humans, the fetal brain consumes 60 per cent of the nutrients the fetus receives, six times as much as that consumed by growing brains in other animals.But until now no one knew how the chemistry of primate placentas might have evolved so as to prompt the placenta to burrow into the uterus wall. Now Laurence Cole of the University of New Mexico in Albuquerque says a sugar-coated placental hormone known as hyperglycosylated chorionic gonadotropin (CG-H) might be responsible. By studying cells taken from primate placentas, he found that animals with more invasive placentas also had more sugars attached to their CG-H molecules, suggesting that more sugars make the placenta more invasive.What's more, the number of sugars present on the CG-H hormone correlates with how evolved an animal is: humans have the most sugars, followed by orang-utans, then Old World monkeys (see Table). Cole says a likely explanation is that CG-H gradually evolved by taking on more sugars - and that this occurred in tandem with the evolution of the brain. "We realised that CG-H evolved at the same time," he says. "It all ties up neatly." Primitive monkeys such as lemurs do not have invasive placentas and do not make CG-H at all."We've never thought about these variants of CG-H as drivers of evolution," says Graham Burton of the University of Cambridge, who studies placental invasion. "It's an intriguing story."As well as possibly giving us our big brains, having an invasive placenta can cause problems. For example, if the placenta doesn't burrow deep enough, the fetus issues chemical distress signals to summon extra blood. This raises the mother's blood pressure, which can cause miscarriage and life-threatening complications such as pre-eclampsia.Cole will present his theory at the international conference on gonadotropins and receptors in Hertfordshire, UK, next week. He will also show that a low level of CG-H in early pregnancy can act as an indicator of miscarriage and pre-eclampsia. Boosting levels of the hormone could lead to ways of treating or avoiding these conditions, he suggests.