Gorilla mother with young
The sea squirt (see picture) is a primitive animal resembling a coffeepot. Its size is up to 12 cm and it lives attached to rocks on the beach or to ships. No trace of a backbone! But the freeswimming larva of this animal has something resembling a backbone, namely a flexible rod-shaped body, called notochord. Along this rod lies a nervous cord. Furthermore the animal is in the possession of an intestinal canal and gills. This larva looks like a primitive vertebrate ...
Suppose that such a larva (not the one of the sea squirt, but another one living more than half a billion years ago) through a mutation had acquired the ability to reproduce itself . In that case the little animal had never reached the adult phase, but it had become the ancestor of a new group of animals with a 'backbone'. The spines and the other bones had not yet evolved but the basic characteristics were present. And there are examples of larvae with the possibility to reproduce. Think of the axolotl, which animal is in fact the larva of a salamander.
The phenomenon that an adult animal or plant shows the characteristics of a juvenile of its ancestor is called pedomorphosis.
It is assumed that man and chimpanzee have a common ancestor because there is only a minor difference between the genomes. But the childhood phase of man is much longer than the one of the chimpanzee. This was probably a major alteration which our ancestor underwent: the childhood was extended. This created the opportunity for the evolution of a larger brain. Because the development could continue during a long time after birth, the proportion of the brain could increase by natural selection.
Pedomorphosis is also demonstrated by the high forehead which man and young apes have in common. This juvenile trait of apes has remained in man.
There are many more kinds of disturbances in the development and most of them have important consequences. It is a source of many macro-evolutionary changes.
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Neoteny