Evolution of snake venom
Molecular studies published beginning in 2006 suggested that venom originated just once among a putative clade of reptiles, called Toxicofera, approximately 170 million years ago.
[10][11] The study therefore suggested that venom had evolved independently in different reptile lineages, including once in the Caenophid snakes.
[8][9] The origin of venom is thought to have provided the catalyst for the rapid diversification of snakes in the Cenozoic period,[21] particularly to the Colubridae and their colonization of the Americas.
[5] Scholars suggest that the reason for this huge expansion was the shift from a mechanical to a biochemical method of subduing prey.
[13] Venom containing most extant toxin families is believed to have been present in the last common ancestor of the Caenophidia, also called Colubroidea.
These toxins subsequently underwent tremendous diversification, accompanied by changes in the morphology of venom glands and delivery systems.
[12] The tubular or grooved fangs snakes use to deliver their venom to their target have evolved multiple times, and are an example of convergent evolution.
The tubular fangs common to front-fanged snakes are believed to have evolved independently in Viperidae, Elapidae, and Atractaspidinae.
[25] Scolecophidia Booidea inc. Pythonidae Acrochordidae Xenodermatidae Pareatidae Viperidae Homalopsidae Colubridae Lamprophiidae Elapidae A cladogram adapted from Fry et al. (2012) showing a subset of suggested protein recruitment events.
Such traditional phylogenies suggested that venom originated along multiple branches among Squamata approximately 100 million years ago: in the Caenophidia, or derived snakes, and in the lizard genus Heloderma.
[29] The single origin hypothesis also suggests that venom systems subsequently atrophied, or were completely lost, independently in a number of lineages.
A study performed in 2014 found that homologs of 16 venom proteins, which had been used to support the single origin hypothesis, were all expressed at high levels in a number of body tissues.
[11] A 2015 study found that homologs of the so-called "toxic" genes were present in numerous tissues of a non-venomous snake, the Burmese python.
Notable examples include 3FTx, ancestrally a neurotransmitter found in the brain, which has adapted into a neurotoxin that binds and blocks acetylcholine receptors.
[21] The change in function of PLA2, in particular, has been well documented; there is evidence of several separate gene duplication events, often associated with the origin of new snake species.
The Elapid snake Bungarus fasciatus, for example, possesses a gene that is alternatively spliced to yield both a venom component and a physiological protein.
[38] PLA2 is thought to have been recruited at least two separate times into snake venom, once in elapids and once in viperids, displaying convergent evolution of this protein into a toxin.
[46] As of 2019, evidence existed both of "overkill" occurring in some lineages, and rapid adaptive evolution, and an evolutionary arms race with prey physiology, in many others.
[53] All these studies suggested a co-evolutionary arms race between prey and predator, indicating another potential selection pressure on snake venom to increase or innovate toxicity.
A 2019 study found that larger body mass and smaller ecological habitats were correlated with increased venom yield.
[47] Another study found that weather and temperature had stronger correlations with snake venom than diets or types of prey.
Current scientific theory suggests that snake venom is not used for defense or for competition between members of the same species, unlike in other taxa.
[41] The natural diets of snakes in the widespread viper genus Echis are highly varied, and include arthropods, such as scorpions, as well as vertebrates.
[64] A 2009 study of the venom of four Sistrurus pit viper species found significant variation in the toxicity to mice.
[17] A 2005 study found that both these lineages have a much simpler set of venom proteins than their terrestrial relatives on the Australian continent, which have a more varied and complex diet.
It has been argued that since sea snakes are typically unable to prevent the escape of bitten prey, their venoms have evolved to act very rapidly.
The activity of proteases, on the other hand, disrupts platelet and muscle function and damages cell membranes, and thus contributes to a quick death for the prey animal.
The study stated that these functions were essentially mutually exclusive; venoms had been selected for based on either their toxicity or their tenderizing potential.
This important adaptation allowed rattlesnakes to evolve the strike-and-release bite mechanism, which provided a huge benefit to snakes by minimizing contact with potentially dangerous prey animals.
The study concluded that these disintegrin proteins were responsible for allowing the snakes to track their prey, by changing the odor of the bitten animal.
