Precocial Versus Altricial
A key dichotomy exists in developmental strategy that affects life-history factors: altricial or precocial. Newborn of precocial species come into the world equipped to cope independently. By contrast, offspring of altricial species are dependent upon parental care to survive, as they are born or hatched in an immature form.
In altricial species embryonic development is relatively rapid. The neonatal brain will grow from its small size after birth. In contrast, development before birth is longer for precocial species and the neonatal brain larger. There is no consistent difference in adult brain size between altricial and precocial species.
Most mammals are small; less than 32 centimeters long.
Humans are large. An early hominid, Australopithecus afarensis, stood 1–1.7 meters, and weighed 60–65 kilos (female–male for both measurements). These proportions persisted for 2 million years, until Homo erectus, 1.5 MYA, with a 1.8-meter height and less sexual dimorphism.
As a group, mammals are relatively precocial. With their altriciality and an unusually big brain, primates are quite the exception.
Throughout descent, hominins lived slow lives. Modern humans extended earlier trends to enhanced altriciality.
Whereas many mammals can walk immediately after birth, it takes human infants up to a year to be able to ambulate on their own. A baby cannot even coordinate its sight with physical manipulation until it is 4 months old.