“Upon your belly you shall go.” Thus did God curse the serpent in the Garden of Eden. The animal that insinuated its way into Eve’s heart had legs, and soon after its creation was condemned to lose its legs and ‘eat dust’, to burrow in the ground. The curse should be interpreted with care. Its outworking was not necessarily straight after its pronouncement, or all of a sudden, and one has to recognise that first God addressed the animal, before going on, in the second part of the curse, to address the spirit that had temporarily possessed it. But the implication is that there was originally just one ‘serpent’ (male and female), that the many different species of snakes today are all related to each other, and that their ultimate ancestor was an animal with legs. So far as Genesis is concerned, creation and evolution are not mutually hostile concepts. Evolution was to follow on from creation.
If the curse was tantamount to a prediction about what we would eventually find in the fossil record, it has proved a remarkably accurate one, confirmed when Michael Caldwell and Michael Lee in 1997 discovered that a snake from the mid Cretaceous, Pachyrhachis, had vestigial hind limbs, though no front limbs. How, back in the 2nd millennium BC, could the writer have known about snakes’ legged origins? Maybe he stumbled across a python’s dried-up skeleton and, noticing the vestiges of a pelvic girdle and posterior limbs, simply made an inspired deduction? Or maybe he split open a fissile rock and came across a fossil snake? Fewer than ten fossil snakes are known with tiny hind limbs, and three, as it happens, are from the Middle East – two from a quarry near Ramallah and a third from Lebanon. Another possibility is that the text had an origin in oral tradition going back to the curse itself.
There are two ways of working out the phylogeny (family tree) of living animals. One is to analyse their genes, the other, to analyse the extent to which they share similarities in form. In either case the results can then be integrated with the fossil record. In principle, the two approaches should be complementary and lead to a consistent account of snakes’ evolutionary origins. That they originated from lizards, and together with lizards form a coherent higher-level group called squamates, has long been known. Indeed, snakes are related not only to lizards but also to geckos, amphisbaenians (worm lizards), iguanas and chameleons – totalling more than 9000 species, arguably the largest group of related terrestrial vertebrates – not to mention the extinct marine reptiles called mosasaurs. However, details of the phylogeny continue to be controversial, mainly because of shared features that criss-cross the family trees. Research over the past two decades has spawned many phylogenies, each claiming to supersede its predecessors on the grounds that it includes more data. The example shown is based on morphological and molecular data, and links the analysis to the fossil record (Simões et al. 2018). The earliest squamate is a lizard of Middle Triassic age from the Italian Alps, called Megachirella. The earliest snakes in the fossil record go back to the Middle Jurassic, from western Europe and North America (Caldwell et al. 2015). Some of their features are ‘derived’, i.e. comparatively modern, hinting that the history of snakes goes back further than the Middle Jurassic. Fossils of mid Cretaceous age have been found in North Africa, the circum-Mediterranean region, North America, Brazil and Argentina. By then the hindlimbs were much reduced and the forelimbs completely lost. Rather oddly, the phylogeny puts true lizards (Lacertidae) next to amphisbaenians: morphology would group the latter with other limbless squamates. In any case, body elongation and limblessness in amphisbaenians and snakes evolved independently. According to one estimate (Wiens et al. 2006), a snake-like body evolved no fewer than 25 times.
The record of snakes is poor, partly because most inhabited terrestrial environments, which were usually places of erosion rather than deposition, and partly because the skeleton is delicate and quickly decays. Those attested mostly occur in coastal or fluvial deposits and appear to have been aquatic, including the hind-limbed fossils from the Middle East (Pachyrhachis, Haasiophis and Eupodophis). These last three are well preserved, but whole fossils are exceptional. The Jurassic fossils consist of skull and jaw elements and a few vertebrae. A lump of mid-Cretaceous amber from Myanmar preserves an almost complete, but headless, baby snake; presumably it formed in an ancient forest. The largest snake known, a Palaeocene- age boid 13 metres long from Colombia, named Titanoboa, consists of a few gigantic vertebrae. One sensationally well-preserved fossil shows a snake in the act of coiling round the eggs of a dinosaur. Characters correlated with head-first burrowing in other animals (e.g. body elongation) are invariably associated with limb reduction. While the habitats of early snakes are debated, there is general agreement that the most recent common ancestor of modern snakes had a small skull with a shape suited to burrowing. The oldest snakes are thought to have been terrestrial but non-burrowing (Da Silva et al. 2018).
Among extant lizards the least evolved are the geckos. As neither geckos nor their closest relatives are poisonous, venom appears to have been a later innovation, restricted to some snakes (with the glands in the upper jaw), bearded lizards, monitor lizards and iguanians (venom glands in the lower jaw). An ‘innovation’ does not entail that venom evolved de novo. Snake venoms are one of the rare instances where a major trait is underlain by a single gene (Barua & Mikheyev 2018). Typically they consist of up to four toxins, which molecular phylogenies indicate have repeatedly been acquired and lost. It seems reasonable to suppose that the genetic code for the toxins was latent in the ancestor of all squamates, and expressed in some snakes and lizards later. Most snakes are non-venomous. .
The largest superfamily is Colubroidea, including colubrids, elapids, vipers and two other families. Colubrids appear in the Oligocene. They had greatly enlarged teeth at the rear of the jaw. In the later-appearing elapids the fangs were at the front of the mouth, where they could be used for striking prey as well as incapacitating prey already seized. The grooves channelling the venom were deeper than with the rear-fanged snakes and met at the edges to form a hollow tube; the venom was injected as through hypodermic needles. Elapids include the cobras, mambas, sea snakes and various coral snakes.
The first elapids were followed by the first viperids. Viper fangs are longer than in the elapids – so long that, when erected, the snake cannot close its mouth. A special rotating bone enables the fangs to be folded up, ready to spring into position when the snake bites. Evolving still further, pit vipers acquired a number of heat-sensitive pits on the front of their face, for seeking out shade by day and detecting warm-blooded prey by night. The Malayan pit viper is sensitive to a temperature change as small as one thousandth of a degree. Boids independently acquired the same feature. Finally, one group of pit vipers – the rattlesnakes – developed a unique warning device at the end of their tail.
Snakes differ from lizards in having a highly mobile skull, an enclosed braincase and no external ear. Some lizards and nearly all amphisbaenians also lack legs. Another sign that snakes were not created in their present form is the loss or reduction of their left lung (in amphisbaenians the right lung). Primitive (least evolved) snakes usually have two functioning lungs, with the left lung about half the size of the right. In the colubrids the left lung has either disappeared or shrunk to a functionless nub, so that the elongated right lung fills much of the body cavity, doing the work of two. Other paired organs, such as the kidneys and in some cases the oviducts, are also reduced from two to one.
A further difference lies in the structure of the eye. Lizards focus by contracting the large ciliary muscles around the eye and either squeezing the lens into a rounder shape or flattening it; they focus by changing the lens’s diameter. Snakes focus by using their enlarged iris muscles to move the whole lens forward or backward and thereby adjusting the focal length. The rationale behind this different method is unclear. They also have no eyelids, instead protecting their eyes with a transparent scale called a ‘brille’.
As conventionally understood, evolution works by the gradual accumulation of tiny accidental genetic mutations that spread through a population because they make the body slightly better adapted to its environment, hence better equipped to leave offspring that pass the mutations on. In reality, that is not what Nature seems to communicate. Indeed, lizards should not have evolved into snakes at all, since losing their legs should have been decidedly disadvantageous – just as if we were to lose our legs. Nonetheless snakes move around with amazing facility. Far from becoming extinct in the great ‘struggle for existence’, they occur on all continents except Antarctica, in cold climates as well as warm, and in a great variety of environments. Some slither forward by creating S-shapes with their bodies; the body undulates from side to side, pushing with the outer edges of the curves against irregularities in the ground. Others use a concertina motion, contracting the front part of the body to bring up the rear, then contracting the rear in order to extend the front. Boas and pythons move by lifting and pushing their belly scales forward, then down against the ground.
Animals in Genesis are categorised according to whether they swim in the sea, fly in the air, walk on the ground, crawl on the ground, or burrow in it. Squamates are perhaps unique in being the one animal group that have adapted themselves to all these environments. This is true of lizards as well as snakes. Some have made a virtue of leglessness by becoming burrowers, going blind in the process. Others have flattened their tails into paddles to become efficient swimmers – even some terrestrial snakes, such as pythons, can swim well. Aquatic lizards include the Cretaceous Adriosaurus, one species of which (suessi) had both forelimbs and hindlimbs, another (microbrachis) only hindlimbs, and today’s marine iguana. Some squamates have defied the curse by developing an ability to climb trees and live above ground. The lizard Draco has membranous wings that enable it to glide. Flying snakes also climb trees. They leap into the air from one branch to another, sometimes over tens of metres, flaring their ribs to double their body width and cutting the air with an undulating motion to increase the air pressure beneath them – the ultimate paradox for such an incapacitated animal, and surely the most compelling demonstration that God can do anything.
Limb loss in snakes and lizards also tells us about limb origins. In all animals, limbs are regulated by Hox genes, switch-boxes that control the expression of entire modules of genes at specified domains along the body axis. At the proper time as an embryo develops, arrays of perfectly co-ordinated algorithms switch on as a package, causing limbs to be lost when the expression of a pre-existing module is inhibited but to grow when expression is allowed to run its course. The relevant genes are the same, whether in man or in mice. Pythons still have most of the genetic software to grow leg bones, but it works only during the first 24 hours after the egg has been laid – after that, the rudiments degenerate, apparently because 17 specific DNA base pairs have been deleted (Kvon et al. 2016). The fossil record, however, shows that the condition was once reversible, prompting an unusual admission that ‘the re-emergence of hindlimbs did not require de novo re-evolution of lost structures’ (Leal & Cohn 2016).
Convergences – features that arise independently in different lineages – support the view that evolution is not a chance process. Limb loss occurs among lizards as well as in all snakes, and has occurred several times even within well-defined lizard subgroups, such as amphisbaenians (one genus of which still retains forelimbs). Body elongation is always accompanied by limb reduction, and usually the forelimbs are lost first. The green tree-python of Papua New Guinea (Morelia viridis) and emerald tree boa (Corallus canina) of South America – species at the end of different evolutionary lineages and separated by an entire ocean – incorporate multiple convergences (Weidensaul 1991). Both are arboreal constrictors that specialise in catching birds, both are lime-green with white markings down the spine, and both rest in the same fashion, with coils wrapped round a narrow branch and the head nestling in the middle. Sometimes the same patterning in different species has the purpose of ‘Batesian mimicry’, mimicking the appearance of an animal that predators know to avoid. In the case of the scarlet king snake, its close resemblance to the venomous coral snake deters bird predators: the scarlet king belongs to the colubrid family, the coral snake to the elapids. Another striking example is heat-sensitive pits, which evolved in boas and several lineages of the python family as well as pit vipers.
Convergence is also encountered at the molecular level. Perhaps the most remarkable instance concerns the finding that agamid lizards have 13 mitochondrial genes in common with snakes, making them appear more closely related to snakes than to iguanians, the group they are most closely related to according to morphology and nuclear genes (Castoe et al 2009). Agamid lizards and iguanans, after all, have legs.