neoteny

In biology, the retention of some juvenile characteristics in an animal that seems otherwise mature. An example is provided by the axolotl, a salamander that can reproduce sexually although still in its larval form.

It has been suggested that new species could arise in this way, and that the human species evolved from its apelike ancestors by neoteny: in human fossil skulls, we see a developmental trend - that facially we resemble a young ape.