How like the kiwi we are

The latest species among mammals, humans, are, surprisingly, more like the avian species than our close relatives—such as chimps—in terms of pregnancy and child-rearing. Most mammals possess a long pregnancy period—around 19 months in the case of elephants—and give birth to almost capable calves who are nearly ready, as soon as they hit the ground, to reach full maturity just by following their mothers. Then why do we, the smartest of them all, have a short—about 40 weeks—but painful gestation and an exceptionally long period for child nurturing?

Evolution answers the question: with an advantage comes its corresponding cost.

Let’s look into avians first. The kiwi, an exceptional bird of the avian class, within its only three-month-long gestation period, lays the largest egg in terms of egg-to-mother size ratio compared to other birds. Obviously, this large egg demands a larger amount of nutrition from the mother’s body, and for that, the kiwi lays only one egg at a time, and the mother becomes significantly weak after pushing most of her nutrition into the egg for her newborn. Furthermore, a short incubation period means a shorter time for the offspring to become mature. Consequently, this interesting bird species's parents spend a considerable amount of time nurturing their offspring and preparing it for the outer world.

Sounds familiar? Yes, this is what we do too. Our infants are so incapable of getting ready for the world by themselves that their mother, father, and sometimes other family members become mandatory for their necessary development. We are similar to Kiwis, aren’t we?

But mammals have developed from this limitation of birds to a more effective and successful long pregnancy period. Then why do humans endure immense pain and a high rate of death during childbirth?

Here comes the evolution part. We call ourselves the smartest for a reason: our enormous brain compared to our body size, an exceptional ratio that no other animal possesses. We are intelligent enough to dominate the planet. But evolution is all about compromise.

With our powerful brains, we have developed two other advantageous traits: firstly, we have more precise hands that are capable of shaping our ideas into tools; secondly, we have managed to walk straight upright to keep our magical hands free to engage in actions. The latter trait has posed an important impact on the lower body part and developed a narrower hip directly below the upper part for perfect balance. This is what our compromise is—to influence earth with our intelligence, first we have to pass a larger and bigger head through a narrower hip.

The more we live in the womb, the larger we grow. As a result, our gestation is only about 40 weeks, which puts an enormous responsibility on both of our parents to take care of us for years before we grow fully mature. This single reason answers the trend of our monogamous society that is different from other mammals’ lifestyle where males tend to mate with as many females as possible.

Humans have walked through evolution’s path, and they have gained the most advanced adaptation of it. But to keep that, interestingly, they have turned back to the behaviors of a pre-mammal class’s members: the birds.

This summary is based on Antone Martinho-Truswell’s essay “How like the kiwi we are” (Aeon), which explores the evolutionary trade-offs in human childbirth and parenting by drawing parallels between humans and the kiwi bird. Main essay link: https://aeon.co/essays/how-evolution-made-humans-more-like-birds-than-other-mammals