External Gestation

If your experience of pregnancy was an overwhelming journey through nine months of symptoms and discomfort you might need to brace yourself for the following:

Humans aren't pregnant for as long as we should be for infant development.

The one difference between homo sapiens and every other mammal is our intelligence. And human intelligence needs a much larger brain. Compared to other primates and animals, the human brain-to-body ratio is significantly larger. A larger brain needs a larger skull. And a larger skull needs a larger exit. But this is where our evolution ran into a problem.

Research has estimated the human foetus probably needs a lot longer in the womb to reach the developmental equivalent of similar species. Let’s have a look at the average gestation:

Human - 266 days

Chimpanzee - 237 days

Gorilla - 257 days

Orang-utan - 260 days

So if a mixed group of higher primates gathered to compare pregnancy experiences, the length of gestation would be relatively similar. And all would take a moment to be thankful they are not these two, the longest gestations on land and in the sea:

African Elephant - 640 days

Sperm Whale - 460 days

However, it is after the birth when the divide appears.

That African Elephant calf, once recovered from the fall of several metres from the birth canal, stands on four legs quickly after birth and walks to the mother’s breasts and begins to feed. Out on savannah, like other herd animals, the young elephant must be able to move with the protection of the herd within hours of birth: no maternity leave for these new mothers!

By contrast, primate newborns only need to cling to the mother’s chest hair and seek the breast. Most will stay attached to the mother for the first half year of life, before learning to move around on their hands and feet. And then as the first birthday approaches, human infants move towards their unique level of development: standing and walking on two legs.

And this is where things get a bit complicated.

At the same time in evolution that humans were increasingly using tools and needing their hands free to carry them (leading to more dependence on upright walking) our brains were getting bigger and bigger. The smarter you are, the bigger the brain. But all this two-legged walking meant changes of its own as pelvises evolved to allow for the change. But the female pelvis was also having to accommodate those bigger infant brains inside bigger infant heads.

So here we are, basically birthing babies while their heads are - just - small enough to fit through the upright human pelvis. It's a tight fit and takes a lot more effort to deliver babies than other species experience. And it’s a lot more dangerous.

The price we pay for survival of the species is the intensive care required in the first year. Human babies are not anywhere near ready for the outside world. From regulating body temperature and breathing to almost constant feeding, the mother’s body is effectively an external womb, picking up where the internal one left off.

Our grandmothers used to talk about “9 months in and 9 months out” which was actually a realistic description of the first degree of care required. While chimpanzee infants typically begin moving about on all fours around 4 months, human babies crawl around 9 months on average.

Added to all of this, our babies are heavier by comparison. About 50% bigger than expected. This means the average newborn weighs around 3kgs, rather than the 2-2.5kg expected for our body size which is actually comparable to female chimpanzee, gorilla and orang-utan. So we have heavier babies who need carrying for longer.

And then we lost most of our body hair!

At this point it does seem like evolution has not necessarily favoured the female human. As we evolved to become hunters able to follow prey for long periods over long distances, the need to stay cool while doing so became important. And a thick coat of body hair did not allow for effective evaporation of cooling sweat. Gradually, the human body became progressively hairless. Ideal for those pursuing prey, not so ideal for those gathering tubers, fruits and leaves while also juggling small infants. It is no wonder baby carriers are one of the earliest human inventions!

So, fast forward to the 21st century: here we are pushing extra-large babies with big heads around an S-bend and through a tube which has a maximum capacity of the average infant skull - just. And then after this we have a prematurely born primate expecting to cling to our non-existent body hair for at least the next six-nine months, with constant access to our breastmilk in place of a placenta and umbilical cord!

And just when you think this is more than enough to be going on with - that evolution which adapted our pelvis for upright walking? And gave us bigger, heavier babies which are harder to push out? Well now our abdominal muscles - designed to support the weight of a pregnant belly swinging as we walk on all fours? Well, that role shifted to our pelvic floor and after holding the baby in for nine months and allowing the baby to be pushed out, now these muscles have the extra load of upright mothers carrying babies on their chest while gravity constantly fights them!

Thanks for that!