The simplification of an animal may be due to the ancestors of that animal having taken to new habits of life. ~ Ray Lankester
Convergent evolution is different than re-evolution. In re-evolution (short for reversion evolution), a genetic expression previously abandoned is re-enabled. The reactivated trait is an atavism: reversion to ancestral type.
The evolutionary throwback of atavism comes from gene conservation. Mathematical modeling suggests that atavism may be had within 6 million years. After 10 million years, an unused gene is unlikely to be functional. Some genes are conserved such that no such expiration applies.
Embryos of various species display ancestral traits. Human fetuses have a tail. Such features typically disappear during development, but an atavism leaves an ancestral feature intact.
In a Lamarckian way, traits that no longer serve a purpose are minimized in time as a process of subtractive adaptation. Gene conservation long leaves the option open for later reversion.
Reversions regularly occur. The snake, which lost its lizard legs and went back toward worm form to get somewhere, is exemplary. Snakes retain the genes for legs. They simply lay dormant.
There are dozens of lizard lineages that have lost their limbs. ~ Australian evolutionary zoologist Michael Lee
Snakes also illustrate that supposed examples of atavism are often facile, in singling out a trait while ignoring others that are invariably coupled to the suite of changes that organisms undergo in adaptive transformation. Snake descent was much more complex than mere reversion evolution.
Snakes are a redesigned organism. ~ American zoologist Stephen Mackessy
Snakes did not lose their limbs in one fell swoop. The first snake with no legs arose 85 million years ago, adapting to be a better burrower. Snake bodies independently evolved at least 26 times.
They had to make so many changes to adapt to life as a tube. ~ American geneticist David Pollock
Losing limbs is the least remarkable thing about snakes. On the inside, snakes underwent extraordinary adaptations. Internal organs pared down, including having only a single lung and liver lobe. To accommodate smooth slithering, snakes gained many vertebrae.
Snake descent was one of the fastest among vertebrates, with an unparalleled number of transformations. It smacks of saltation.
Snakes metabolism ratchets to the greatest extremes of any vertebrate. This owes to snakes having to swallow their meals whole, as they lack the means to tear their food into pieces.
Snake jaws, and the head, underwent numerous adaptations to permit swallowing animals many times its size. The skin around the mouth is usually folded and only stretched when wide open.
Snake mouths can open so far as to rip apart the interlocking proteins that hold jaw muscles together. These proteins repair themselves after feeding.
When a snake swallows its meal, its digestive system gears up for the feast. The small intestine and liver may double in size. Heart and kidneys also bulk up big-time.
Metabolic rate may jump to 45 times its normal pace. This may last for days. Meanwhile, a snake is motionless: its energy consumed in consuming.
After digestion, which may take 2 weeks, organs return to their between-meal norm. In species that feed sporadically, such as the python, the entire intestine becomes inactive between meals.
The difference in metabolism between a live snake and a dead snake is minimal. ~ American biologist Todd Castoe
Conservation of energy lets snakes go many moons without needing to eat; thereby letting snakes be highly opportunistic in their hunting. Snakes cope well with food scarcity: the great advantage of ectothermy.
Being able to ramp up metabolism to cope with a feast is one thing. Catching those outsized meals is quite another.
Some snakes, such as boas and pythons, grew large enough to put the squeeze on their prey. Another line of snakes developed a different device to bring down a large lunch: venom.
In a sense, snake venom was reversion evolution, as it revitalized a genetic art 200 million years old. Venom first developed in an ancestor of lizards that found a poison fang advantageous.
Evolving venom involved more than turning saliva toxic. Snakes recruited proteins from other parts of the body, such as those used to clot blood or regulate blood pressure. These were mutated into potency and concentrated to catastrophe for those that would be prey.
The sophisticated fangs of snakes was a later innovation: within the past 80 million years. Rear fangs with grooves for venom flow evolved into hypodermic needles up front, along with muscles to pump the poison.
Advanced venoms, which may be a complex of over 100 compounds, may do more than kill. Some serve as a scent trail, helping a snake locate a bitten animal that manages to get away.
Rattlesnakes, which often live in deserts and other sweltering biomes, pack their venom with proteins that break down the tissues of those bitten. This lets them win the race between digestion and rot.
Snake venom also serves defensive purposes. Compounds for predator deterrence include those that elicit extreme pain in their victims – if not dead: once bit, twice shy.
In a coevolutionary competition, prey animals may evolve resistance to snake venom. That is not possible in the instance of Australian tiger snakes, which perfected their poison 10 million years ago and have conserved its formula over that vast expanse of time. Tiger snake venom is killer.
Tiger snakes and their close relatives have toxins that are almost identical. The toxins target a part of the blood clotting cascade that is the same across all animals. ~ Australian herpetologist Bryan Fry
Snake adaptations varied in several ways. Sea snakes, which arose 20–9 MYA, convergently evolved greater sensitivity to their environment via modified scale sensilla. These sensilla are small, tactile, mechanosensory organs located on the head scales of many squamates.
The scale sensilla of sea snakes are much more dome-shaped than the sensilla of land snakes, with the organs protruding further from the animals’ scales, making them able to sense vibrations from all directions. The scale sensilla on some of the fully aquatic snakes cover a much higher proportion of the scales’ surface. Sea snakes use these organs to sense objects at a distance by ‘feeling’ movements in the water. This hydrodynamic sense is not an option for land animals. In water, a new way of sensing the environment becomes possible. ~ Australian herpetologist Jenna Crowe-Riddell
Numerous eyeless animals, including the Kauai cave wolf spider, live in total darkness. Lack sight, other senses developed heightened sensitivities to compensate. As with other animals, such as blind cavefish, these spiders descended from ones with eyes.
The atavistic resurrection of complex ancestral traits, contrary to Dollo’s law, appears to be more frequent than commonly thought. ~ German zoologist Katja Domes et al
Mites have been mighty fond of devolution. Free-living house dust mites descended from parasites which had evolved from free-living organisms.
More specifically, house dust mites speciated from mites that live in animal nests. Dust mites abandoned parasitism via digestive adaptation that allowed them to live off keratinous material, such as hair, feather quills, and flakes of skin.
Oribatids are an order of slow-growing mites with low metabolism. Many are soil saprovores. Some live in trees. Others are predatory. None are parasitic.
Some oribatid mites went from sexual reproduction to parthenogenesis before going back to having sex. Parthenogenesis predominates in soil mites, whereas sexual species colonize the bark of trees and mosses. Arboreal mites have a tougher time getting enough to eat, and so the diversity that comes from sex helps maintain populations.
Viruses mediate gene transfer between cells and crucially enhance biodiversity. ~ Argentinian biologist Gustavo Caetano-Anollés et al
In unifying all other life to the DNA standard, viruses were the universal common ancestor. This demonstrated an unsurpassed genetic knowledge in knowing what makes cells tick.
Viruses typically travel light: keeping only the tools they need. Most lack reproductive machinery and even the means to metabolize. There are exceptions.
Giant viruses have incredible machinery that seems to be very similar to the machinery that you have in a cell. ~ Gustavo Caetano-Anollés
The giant mimivirus has sufficient genomic instructions and facilities to run its own business. It simply saves labor by employing the machinery of its amoeba host to effect reproduction.
The viral factories of the mimivirus are spectacular. Their size is similar to the size of the nucleus of the virus host. ~ Patrick Forterre
The miniaturization of viruses may be the earliest example of reversion evolution. Having architected the physical language of life (DNA), viruses settled into simplicity by dropping inessentials.
Viruses are stoic: accepting the vicissitudes of existence while quickly adapting to meet the demands of the moment. They travel the world and stay in hotels rather than bothering with owning a home. Lacking metabolism saves cooking for one. Surreptitiously entering and sneaking past sentries is a more exciting and demanding lifestyle than foraging by a wide measure. In enjoying a truly global community and reliant upon other life, viruses are the undisputed Zen masters of wabi-sabi living.
Simplicity is the ultimate sophistication. ~ Leonardo da Vinci