What came first? Bigger brains or lots of sex?

November 20, 2002

LOW fertility and frequent pregnancy complications may be the price that we have paid for evolving a large brain.

For the fetus to get enough nutrients to grow a hefty brain the placenta has to aggressively invade a mother's uterus, says a new theory. But that can also provoke her immune system, causing dangerous complications.

However, recent research suggests that exposure to a man's semen helps a women's immune system prepare for pregnancy (New Scientist, 9 February, p 32). So low fertility in humans reduces complications during pregnancy by giving a woman's immune system more time to adapt. Human fetuses spend 60 per cent of their energy on their brain, 3 times as much as other mammals. Twenty weeks into pregnancy, the placenta attacks the uterine wall for a second time, burrowing in more deeply than in any other mammal.

But burrowing deeper is risky. It can provoke the mother's immune system to attack the placenta, which is loaded with foreign genes from the father. This can trigger pre-eclampsia, where the placenta leaks toxins into the mother's circulation, causing blood pressure to spike dangerously. Within hours it can escalate into kidney failure, brain haemorrhaging and death.

It is thought that humans are the only mammals to suffer frequent pre-eclampsia, which occurs in 3 per cent of pregnancies. We are also far less fertile: a bitch that mates just once when it is on heat usually gets pregnant, yet women typically take six months to conceive.

Research by Pierre-Yves Robillard, a neonatologist at Sud RŽunion Hospital on the Indian Ocean island of RŽunion, has shown that women who have sex with the father for over a year before getting pregnant have a 5 per cent chance of developing high blood pressure and pre-eclampsia compared with a massive 40 per cent chance for those who have only been having sex with the father for four months or less.

Robillard is now proposing that this is why we are less fertile- the extra sex gives women a better chance of surviving the placental invasion. "If we had kept the same fertility as other mammals, we would have pre-eclampsia rates of 20 per cent," he told a workshop about pre-eclampsia in Mauritius. "Humans could not have survived."

The theory has generated both interest and scepticism. "It's an interesting idea that placental invasiveness has something to do with brain expansion," says David Haig, an evolutionary biologist at Harvard University, "but other possibilities can't be eliminated." For example, pre-eclampsia may have become more common as societies became better at caring for ailing mothers and babies.

And Robert Martin, an anthropologist at The Field Museum in Chicago, questions whether invasive placentas are linked to larger brains. "Dolphins have a non-invasive placenta," he says, "yet the next biggest brain sizes after humans are found in dolphins."
Author: Douglas Fox

New Scientist issue: 23rd November 2002


"These articles are posted on this site to give advance access to other authorised media who may wish to quote extracts as part of fair dealing with this copyrighted material. Full attribution is required, and if publishing online a link to www.newscientist.com is also required. Advance permission is required before any and every reproduction of each article in full - please contact angela.bourton@rbi.co.uk. Please note that all material is copyright of Reed Business Information Limited and we reserve the right to take such action as we consider appropriate to protect such copyright."

UK CONTACT - Claire Bowles, New Scientist Press Office, London:
Tel: 44-207-331-2751 or email claire.bowles@rbi.co.uk

US CONTACT - Michelle Soucy, New Scientist Boston Office:
Tel: 617-558-4939 or email michelle.soucy@newscientist.com

New Scientist

Related Immune System Articles from Brightsurf:

How the immune system remembers viruses
For a person to acquire immunity to a disease, T cells must develop into memory cells after contact with the pathogen.

How does the immune system develop in the first days of life?
Researchers highlight the anti-inflammatory response taking place after birth and designed to shield the newborn from infection.

Memory training for the immune system
The immune system will memorize the pathogen after an infection and can therefore react promptly after reinfection with the same pathogen.

Immune system may have another job -- combatting depression
An inflammatory autoimmune response within the central nervous system similar to one linked to neurodegenerative diseases such as multiple sclerosis (MS) has also been found in the spinal fluid of healthy people, according to a new Yale-led study comparing immune system cells in the spinal fluid of MS patients and healthy subjects.

COVID-19: Immune system derails
Contrary to what has been generally assumed so far, a severe course of COVID-19 does not solely result in a strong immune reaction - rather, the immune response is caught in a continuous loop of activation and inhibition.

Immune cell steroids help tumours suppress the immune system, offering new drug targets
Tumours found to evade the immune system by telling immune cells to produce immunosuppressive steroids.

Immune system -- Knocked off balance
Instead of protecting us, the immune system can sometimes go awry, as in the case of autoimmune diseases and allergies.

Too much salt weakens the immune system
A high-salt diet is not only bad for one's blood pressure, but also for the immune system.

Parkinson's and the immune system
Mutations in the Parkin gene are a common cause of hereditary forms of Parkinson's disease.

How an immune system regulator shifts the balance of immune cells
Researchers have provided new insight on the role of cyclic AMP (cAMP) in regulating the immune response.

Read More: Immune System News and Immune System Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.