<P> A putative cause for the broad changes seen in domestication syndrome is pleiotropy . Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits . Certain physiological changes characterize domestic animals of many species . These changes include extensive white markings (particularly on the head) floppy ears, and curly tails . These arise even when tameness is the trait under selective pressure . The genes involved in tameness are largely unknown, so it is not known how or to what extent pleiotropy contributes to domestication syndrome . Tameness may be caused by the down regulation of fear and stress responses via reduction of the adrenal glands . Based on this, the pleiotropy hypotheses can be separated into two theories . The Neural Crest Hypothesis relates adrenal gland function to deficits in neural crest cells during development . The Single Genetic Regulatory Network Hypothesis claims that genetic changes in upstream regulators affect downstream systems . </P> <P> Neural crest cells (NCC) are vertebrate embryonic stem cells that function directly and indirectly during early embryogenesis to produce many tissue types . Because the traits commonly affected by domestication syndrome are all derived from NCC in development, the neural crest hypothesis suggests that deficits in these cells cause the domain of phenotypes seen in domestication syndrome . These deficits could cause changes we see to many domestic mammals, such as lopped ears (seen in rabbit, dog, fox, pig, sheep, goat, cattle, and donkeys) as well as curly tails (pigs, foxes, and dogs). Although they do not affect the development of the adrenal cortex directly, the neural crest cells may be involved in relevant upstream embryological interactions . Furthermore, artificial selection targeting tameness may affect genes that control the concentration or movement of NCCs in the embryo, leading to a variety of phenotypes . </P> <P> The single genetic regulatory network hypothesis proposes that domestication syndrome results from mutations in genes that regulate the expression pattern of more downstream genes . For example piebald, or spotted coat coloration, may be caused by a linkage in the biochemical pathways of melanins involved in coat coloration and neurotransmitters such as dopamine that help shape behavior and cognition . These linked traits may arise from mutations in a few key regulatory genes . A problem with this hypothesis is that it proposes that there are mutations in gene networks that cause dramatic effects that are not lethal, however no currently known genetic regulatory networks cause such dramatic change in so many different traits . </P> <P> Feral mammals such as dogs, cats, goats, donkeys, pigs, and ferrets that have lived apart from humans for generations show no sign of regaining the brain mass of their wild progenitors . Dingos have lived apart from humans for thousands of years but still have the same brain size as that of a domestic dog . Feral dogs that actively avoid human contact are still dependent on the human niche for survival and have not reverted to the self - sustaining behaviors of their wolf ancestors . </P>

What led to domestication of animals in mesolithic age