A recent research report reveals the kind of processes that ammonite atavism might have involved1. This research investigated how birds react to altitude, specifically with regard to their ancestral environment. Domestic chickens were introduced to high altitude on the Tibetan plateau over 1000 years ago. As one might expect, the modern descendants of those pioneer birds have evolved adaptations enabling them to survive successfully at these very high elevations. For example, one adaptation to the low atmospheric oxygen levels (hypoxia) is a distinct form of haemoglobin in their embryos’ blood that is unusually efficient in carrying oxygen to their developing tissues. These adaptations are now innate to the high altitude birds; they are permanently present and heritable.
Chickens from low altitude areas are far less able to live in hypoxic conditions (for example their eggs have reduced hatch rates, and hatchlings have a low rate of survival to adulthood). Some survival is possible, however, due to physiological changes the low altitude chickens undergo when oxygen is low. Those changes are neither permanent nor hereditary, they involve a set of genes that change the degree to which they are active (i.e. their “expression patterns”) depending on the amount of available atmospheric oxygen.
The adaptations of the high altitude birds are mechanistically quite separate from the flexible physiological response of the low altitude birds to oxygen availability (they involve different genes). A fact that is perhaps surprising is that the high altitude birds, in addition to their unique, special adaptations to altitude, have retained the original system as well. Experiments have shown that, if eggs from high altitude birds are artificially transferred to low altitude conditions, the flexible system still reacts, setting the physiology of the embryos to the ancestral low altitude state. The high altitude chickens therefore have two biological systems for dealing with altitude: the original reactive one, and the more recently evolved one that provides their increased capability.
It appears that the flexible system adjusts to meet either low or high altitude conditions (albeit to a limited extent regarding the latter) suggesting that both oxygen abundance and deficiency present their own physiological challenges. Though the high altitude birds do not normally encounter abundant oxygen concentrations, they have retained an ancestral process in a dormant state and can reactivate it as a resource to adjust to a high oxygen situation when they are exposed to it. This, then, is a form of atavism that provides a resource when an environmental challenge resembles an ancestral environment.
”This, then, is a form of atavism that provides a resource when an environmental challenge resembles an ancestral environment.”
The ammonite and chicken examples differ in their evolutionary extent, one being truly evolutionary in terms of time and biology, the other an example, over a much shorter term, of individual adaptations. Both suggest biological memory of an ancestral environment. Though the two examples almost certainly differ in the detail of their genetic and molecular basis, the bird example demonstrates atavism mechanistically, and illustrates molecular processes that may resemble those that contributed to atavism in ammonites.
Thus, innate, possibly dormant, processes can be a resource for biological response to environmental challenge. This is inconsistent with the widely held view of an evolutionary mechanism in which evolution depends solely on adaptations being generated by a random process. As suggested earlier, a process in which biological solutions are generated at random seems unlikely to have led to atavism repeatedly being the solution to the extreme environmental conditions that caused mass extinctions of ammonites. The implication is that organisms faced with changing environmental conditions can depend on more than chancy randomness; other phenomena might supplement random mutation in the generation of variation, perhaps in ways that enhance the generation of variations that match environmental opportunity or threat. There are, in fact, ways in which organisms generate adaptation through direct and specific reaction to changes in their environment.
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References
- Ho, Wei-Chin, Diyan Li, Qing Zhu, and Jianzhi Zhang. ‘Phenotypic Plasticity as a Long-Term Memory Easing Readaptations to Ancestral Environments’. Science Advances 6, no. 21 (1 May 2020): eaba3388. https://doi.org/10.1126/sciadv.aba3388 [↩]