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^[1].

An example can best illustrate the concept. 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.
If the onset and offset are unaffected, but the rate of growth is increased, the tail will be larger. If the offset is delayed and the rate is unaffected, the tail will be also larger. All possible combinations have been identified in living animals.

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

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^[2].

[edit] References

1. ^  Horder, Tim (April 2006) Heterochrony. In: Encyclopedia of Life Sciences. John Wiley & Sons, Ltd: Chichester.
2. ^  See "Comparison of cranial ontogenetic trajectories among great apes and humans" by P. Mitteroecker et al. in Human Evolution (2004) Volume 46, pages 679-697 Entrez Pubmed 15183670 and also "Ontogenetic study of the skull in modern humans and the common chimpanzees: neotenic hypothesis reconsidered with a tridimensional Procrustes analysis" by X. Penin, C. Berge and M. Baylac in American Journal of Physical Anthropology (2002) Volume 118, pages 50–62. Entrez Pubmed 11953945.

[edit] See also

• Developmental biology
• Embryogenesis
• Evolutionary biology
• Neoteny
• Pedomorphosis
• Progenesis
• Phylogeny