This is the first of two science articles that examine the evolution of snakes.
You may also wish to read Homer Simpson and Predator Snakes.
We now know that all snakes evolved from lizards; lizard ancestors of today’s snakes lived during the Cretaceous era some 120 million years ago.Close cousins of snakes living today include Nile Monitor lizards…
and Komodo Dragons.
The evolutionary process started when Cretaceous lizards found ready sources of food underground. They reached these sources of food by burrowing and searching in tunnels and narrow crevices. Cretaceous lizards adapted to life underground where they could successfully hunt and where they were safe from predators.
One lizard alive today that may have resembled those early burrowing lizards is the Earless Monitor lizard of Borneo. Earless Monitor lizards have reduced eyes and limbs, scaly bodies and no external ears. They are burrowing lizards that eat earthworms and other similar prey.
1) Proto snakes evolved very long backs
Proto snakes spent much of their lives underground hunting insects and other prey in narrow tunnels. Crawling on all four limbs was cumbersome and slow!
Proto snakes stopped using their limbs; they evolved elongated backs and streamlined bodies to help them slither and wriggle through narrow tunnels.
Snakes are now so long that some species living today have 500 vertebrae in their backs! (Humans only have 33 vertebrae!)
2) Proto snakes grew small limbs
In the Cretaceous era the evolutionary pressures were for snakes to grow smaller limbs. ‘Slithering’ in narrow tunnels and crevices meant that limbs often impeded their locomotion, slowing them down as they tried to catch prey.
A lizard species from South Africa shows what the bodies of those proto snakes might have looked like. This tiny lizard has evolved a long back to help it slither across tufts of grass instead of through narrow tunnels. Notice its short front limbs.
3) Proto snakes lost the ability to see
Proto snakes living in continuous darkness in underground tunnels had no need for eyes. To prevent soil entering their eyes and damaging them they kept their eyes permanently closed. Their eyelids eventually fused together so they could no longer open them.
Some snake descendants of those underground proto snakes have never regained the ability to see. The Texas Blind snake has two dark spots which represent where the eyes of its ancestors used to be. It lives underground feeding on termites and ants.
4) Proto snakes regained the ability to see
Around 100 million years ago, after spending 20 million years underground, proto snakes began to live above ground. The Najash rionegrina, with its two rear vestigal limbs, was one such snake.
Living above ground meant that snakes needed to see.
The solution was simple but effective; snakes developed transparent eyelids (called ‘spectacles’) which gave them the ability to see. This image of a Checkered Keelback snake shows transparent, fused eyelids covering its eyes. Its eyelids are permanently shut but it can still see, although it cannot blink.
When a snake is ready to shed its skin, its spectacles become visible when they mist over and become cloudy.
Snakes shed their spectacles and scaly skins anything from once to four times a year.
Some snakes have developed the amazing ability to see ‘heat’ thanks to special heat seeking ‘pit organs’ on their faces.
‘Pit’ organs allow snakes to ‘see’ the infrared spectrum of light. They can detect the warm bodies of prey in deep undergrowth that would otherwise be hidden from view.
These images show what ‘snake prey’ might look like when seen through the infrared spectrum of light.
5) Proto snakes inherited scales from their lizard ancestors
Snakes inherited scales from their lizard ancestors more than 120 million years ago. Scales perform various functions for snakes such as protecting soft body tissue…
helping them slither across smooth surfaces….
and providing them with camouflage.
Scales even help snakes capture prey. The aquatic Little File snake from Australia uses its rough scales for grip as it wraps its body around slippery fish when hunting in rivers.
The rattles of Rattle snakes are made of modified scales. Rattle snakes use their tails to sound a warning to approaching predators.
Snake scales are made of a tough protein called keratin, the same protein that human nails are made from!
6) Proto snakes developed a BIG appetite!
Snakes have evolved the amazing ability to eat a few big meals instead of lots of small ones. In the snake world eating a BIG meal means consuming the prey WHOLE.
Snakes are able to eat prey much larger than themselves. The skin around their mouths has folds allowing their mouths to expand.
The muscles in their jaws can also stretch really wide.
The bones in their skulls have evolved adaptions which allow snakes to eat these massive meals.
The two halves of snakes’ lower jaws can move independently of each other. Quadrate bones give their jaws the extra flexibility that they need to open their mouths really wide.
Rear facing teeth means that prey cannot wriggle free and jump out of their mouths after being caught. Snakes do not use their teeth to chew food.
When an African Rock python swallows a large meal like an impala deer ….
…some of its internal organs DOUBLE in size. Organs that grow bigger include the heart, kidneys, liver and intestine.
Its heart has to pump extra blood into other organs (kidneys, small intestine and liver) to help the python digest its meal. Its heart also has the amazing ability to move to one side to allow swallowed prey to pass through its food pipe.
You can see that the internal organs are not side by side; they are staggered one behind the other so that they fit into their long, narrow bodies. Many snakes only have one fully functioning lung.
This python in this image has just eaten a sheep! It will remain motionless for two weeks as it digests its meal. Although it remains completely still its metabolic rate will increase a remarkable 45 times!
A python expends the same amount of energy digesting massive meals as horses galloping on race tracks!
After two weeks of remaining completely still the python will have fully digested its meal; it now goes into almost complete shutdown reducing its metabolic rate to the lowest of any known vertebrate.
As a result of drastically reducing the amount of energy it uses, it will now be able to survive for months before its needs to hunt once again.
All snakes are carnivores; there are no known species of herbivorous snake. In contrast, there are many species of herbivorous lizard such as this seaweed eating Marine iguana from the Galapagos Islands.
After snakes moved above ground 100 million years ago they became more vulnerable to predators. Infrequent meals reduces the risks of being eaten when they are out in the open searching for food.
Nevertheless snakes still have many predators including Secretary birds….
and Honey badgers.
7) Snakes flick their tongues
Like many lizards snakes have a peculiar habit (peculiar to us humans, that is!) of flicking their tongues in and out of their mouths as they slither around.
Snakes use their forked tongues to collect chemicals from the air or ground so they can smell them. Snake tongues have no taste buds and by themselves snake tongues can neither smell or taste. The actual ‘smelling’ takes place when the snake transfers chemicals from its tongue onto the Jacobson’s organ at the roof of its mouth.
The ability to detect chemical signals allows snakes to follow both prey and potential mates.
8) Snakes hear with their inner ears and jaws
Snakes have no external ears but they do have inner ears. Snakes’ inner ears allow them to hear low frequency airborne vibrations such as the sound of prey running through the grass.
As well as hearing airborne vibrations snakes can detect ground borne vibrations through their jaw bones.Their jaw bones pick up the ground borne vibrations of approaching prey and predators. These vibration signals are then transmitted to their brains for processing. Snakes will often lie low with their jaws touching the ground to increase their chances of detecting ground borne vibrations.
Watch this human acting like a snake!
Look at this human evolving into a snake!
You could research one of the following original science fair project ideas: