Heterochrony

In biology, heterochrony is defined as a developmental change in the timing of events, leading to changes in size and shape. There are two main components, namely (i) the onset and offset of a particular process, and (ii) the rate at which the process operates. A developmental process in one species can only be described as heterochronic in relation to the same process in another species, considered the basal or ancestral state, which operates with different onset and/or offset times, and/or at different rates. The concept was first introduced by Ernst Haeckel in 1875.

An example can best illustrate the three dimensions of heterochrony. Combinations thereof have been identified in living animals:
 * If a developmental process, such as the growth of a tail in the embryo of "species A", starts earlier and ends earlier than that of "species B", but the rate of growth is the same for both, the final result may basically be the same, although the tail of species A develops earlier than the one of species B. The earlier exhibits predisplacement, and the later species exhibits postdisplacement.
 * If the rate of growth is increased, and the time between the start and end of development is decreased proportionally, the tail will end up the same size. The species with faster growth exhibits acceleration, and the species with slower exhibits neoteny.
 * If the end of development is delayed and the rate is unaffected, development progresses further, and the tail will be also larger. The species that develops further exhibits hypermorphosis, and the species that does not develop as far exhibits progenesis.
 * If the onset and completion times of growth are unaffected, but the rate of growth is increased, the tail will be larger. This is a combination of hypermorphosis (because more growth occurs) and acceleration (because the growth is faster).

Heterochronies are easily identifiable when comparing phylogenetically close species, for example a group of different bird species whose legs differ in their average length.

Humans
Several heterochronies have been described in humans, relative to the chimpanzee. For instance, in chimpanzee fetuses brain and head growth starts at about the same developmental stage and present a growth rate similar to that of humans, but end soon after birth. Humans, on the contrary, continue their brain and head growth several years after birth. This particular type of heterochrony is named hypermorphosis and involves a delay in the offset of a developmental process, or what is the same, the presence of an early developmental process in later stages of development. In addition, humans are known for presenting about 30 different neotenies in comparison to the chimpanzee.

Populations
People of Asian and Hispanic descent reach full skeletal maturity significantly earlier than individuals who are of African or European descent, resulting in an average smaller final height and a slightly different pattern of skeletal maturation. Genetic differences, diet, and nutritional intake may influence variations in the bone growth pattern.