Why We Only Have Ten Toes (It's a Long Story)

Around 380 million years ago, at the tail end of the Devonian Period, strange creatures began to emerge from the shallow seas.

The Devonian is often known as “The Age of Fishes” - the time when things like Dunkleosteus ruled the waves.

But then came a surprising transition—some of the fishes started moving onto land.

One of the earliest of these new creatures was a tetrapod named Acanthostega.

It lived about 365 million years ago, and it looked a bit like a weird mashup of a fish and a land animal.

It had a tail fin shaped like an oar, short ribs that ran the length of its body, and gill bars like a fish.

But it also had four limbs and eight fingers and toes on each one.

Today, all mammals from humans to bats have five fingers or fewer.

Yes, even whales, whose finger bones are hidden in their fins.

Birds and reptiles have four or fewer.

And amphibians get the best of both worlds, often having four digits on their “hands” and five on their “feet.” But no species of vertebrates have more than five digits, let alone eight.

So how did Acanthostega manage it?

And why is it that by 348 million years ago, five fingers and toes had become the norm among the new tetrapods?

If five fingers and toes are good, why isn’t six or seven better?

Can we ever get those lost digits back?

The story of how and when the tetrapods lost their toes is an ancient one that we’re still filling in the gaps of - but it looks like it has something to do with the early tetrapod strut.

The end of the Devonian was a period of dramatic changes.

There were two mass extinction events - one at 374 million years ago and another at 359 million years ago - which led to losses of at least 50 percent of biodiversity.

This would’ve left a lot of niches open for exploitation—which early tetrapods were able to do, in part by leaving the oceans for the land.

But this would take some big changes to their bodies.

They’d have to trade gills for lungs, for one thing.

And a new way of breathing wasn’t the only thing tetrapods would need.

They would also have to develop new methods of seeing, hearing, feeding, and moving.

Like breathing, getting around is a problem that impacts a lot of different parts of an animal’s physiology.

First, there’s gravity.

Fish don’t have to worry about it so much being in the water, but to come on land, tetrapods needed stronger ribs and vertebrae to keep all their organs from moving around too much or collapsing.

They also needed new limb supports — a more robust humerus to support the arm, and a pelvic girdle to support the legs.

These are the structures that help the limbs move independently from the spine.

The pelvic girdle in fish isn’t attached to the vertebrae and is generally small.

And we start to see changes to this with Acanthostega, which had a pelvic girdle attached to its backbone with soft tissues.

But its bones still weren’t very robust, which is why some scientists doubt that it had the ability to really lift its back end off the ground.

Living alongside Acanthostega was Ichthyostega -- and it was one weird tetrapod.

Ichthyostega had a more robust femur and pelvic girdle compared to its neighbor.

Thanks to well-preserved skull fossils, scientists know that it had recurved teeth for hunting, and the ear region of its skull suggests that it was adapted to hearing underwater.

But its hindlimbs are where things get really strange.

Icthyostega had seven digits on each one.

And they were arranged in a very unusual pattern.

Three small digits extended at an angle from the other four larger digits—almost like it held its toes in a permanent Vulcan salute.

At least three different fossils show this structure, which seems to indicate that the bones were in that position when they fossilized.

They weren’t moved that way by rocks or debris.

Scientists think the digits must have been held that way by strong tissues in the feet.

To try to figure out how Ichthyostega got around, scientists created a digital 3D model of the critter and ran tests to see how its bone structure would support different forms of locomotion.

They found that the bone structure of its back limbs probably meant that Ichthyostega couldn’t put its feet flat on the ground, and that they were more like paddles.

Some researchers think that Icthyostega moved its limbs in parallel - first both right limbs, then both left limbs - rather than alternating limbs, like modern quadrupeds.

Others think it moved like a mudskipper - propelling its body forward using its pectoral fins, kinda like a human using crutches.

We don’t know whether Ichthyostega and Acanthostega had webbing around their digits, but it would make sense, given that they lived in semi-aquatic environments.

Their locomotion was probably a mixture of swimming and some early form of walking.

Things would be a lot easier if we could actually /watch/ one of these tetrapods walk around.

While we can’t bring these extinct animals back to life à la Jurassic Park, we do have some clues that can help us figure out what early tetrapods were doing: fossilized footprints.

The study of these trackways is called ichnology, and there are nine different trackways from three locations in Ireland that are all older than the oldest tetrapod body fossils we’ve found.

The nine different trackways include hundreds of individual prints of varying sizes.

The different sizes could mean different species of animal were walking around, or that the prints captured different sizes of the same tetrapod.

Scientists don’t know exactly what animals made the prints, but they can learn a lot about their locomotion from them.

And these prints clearly show that the tetrapods were walking with both front and hind limbs, not dragging half of their body on the ground.

But, unfortunately, none of the prints show any sign of digits.

This could be due to a couple of different things.

Maybe the ground was too soft, and the individual digit prints blurred together.

Or maybe the digits were connected by webbing.

What’s clear is that these tetrapods were walking on all fours, not paddling or flopping.

Does that mean scientists have misunderstood how Ichthyostega and Acanthostega moved?

Or were there other tetrapods walking around and we just haven’t found anything in the fossil record yet?

It’s hard to know.

What we can say is that, by 348 million years ago, we have fossils of creatures with five digits.

Pederpes, which lived 348 million years ago, and Casineria, from 340 million years ago, were essentially fully terrestrial quadrupeds.

While Acanthostega and Ichthyostega had very inflexible shoulders and wrists, with limbs that probably pointed straight out from their body, Pederpes had bones that seem to suggest that its feet pointed forwards rather than to the side.

The evidence is even more striking with Casineria, which was only about the size of a mouse, but had five fingers and toes with claws on each limb.

This probably means that its feet were now fully adapted for walking on land over different surfaces rather than spending most of its time in the water and occasionally coming onto land.

As to why five seemed to be the maximum magic number when it came to digits, well, we’re just not sure yet.

But researchers hypothesize that it has to do with the pressure placed on the small bones of the wrist and ankle joints when an animal has to do more with its limbs than just paddle.

The wrist bones of Acanthostega weren’t arranged in a formation that would’ve made walking a very easy motion.

And maybe there’s too much pressure on those small bones or just not a large enough range of motion for getting around on land when an organism has more than five fingers.

But there isn’t enough evidence to really test this hypothesis yet, because we haven’t found enough of these early tetrapod fossils.

What we do see is that creatures that show up after Ichthyostega and Acanthostega generally don’t regain seven or eight digits.

So why not?

Well, it might be explained by a thing called Dollo’s Law.

This suggests that when complex characters in the body are lost, they aren’t regained, because the genetic features that resulted in that loss are highly unlikely to reoccur.

Basically, it’s easier to lose a trait than it is to regain it.

An example would be that modern whales don’t have legs, and modern birds don’t have teeth, despite what we might have seen in older species.

In the case of having more than five digits, it’s rare to see that happen again in the fossil record—although it has happened.

In 2003, researchers found a fossil of a marine reptile from the Triassic Period, 242 million years ago, with seven digits on its front limbs and six on its hind limbs.

The researchers believe this was an example of convergent evolution, and that the organism in question would’ve led a largely aquatic life like Icthyostega and Acanthostega.

Maybe in those conditions, it’s more advantageous to have more fingers.

But it’s been nearly 400 million years since the tetrapods first crawled onto land—anything can happen, right?

Well, maybe.

Some lab experiments investigating digit development suggest that the genes that control it probably also play important roles in the development of other traits.

And this might be a reason that we’ve never regained those lost digits - too many other parts of the body would also be affected.

So no matter how convenient it might seem to have eight fingers for extra multi-tasking, we probably won’t have to make an episode about how the tetrapods regained their toes anytime soon.