Humans developed from crocodile-like animals which developed enormous eyes enabling them to see food on land, a new study says.
Crocodile-like animals first saw easy meals of spiders, millipedes and centipedes on terra firma and later evolved legs to walk the Earth, the study shows.
Professor Malcolm MacIver, of Northwestern University in Illinois, said: “Why did we come up on to land 385 million years ago?
“We are the first to think that vision might have something to do with it.”
Neuroscientist Prof MacIver and evolutionary biologist Prof Lars Schmitz studied the fossil record and discovered eyes nearly tripled in size before – not after – the water-to-land transition.
The trebling coincided with a shift in location from the side of the head to the top.
The expanded visual range of seeing through air could have eventually led to larger brains in early terrestrial vertebrates.
This would have also helped the ability to plan and not merely react – as fish do.
Prof MacIver said: “We found a huge increase in visual capability in vertebrates just before the transition from water to land.
“Our hypothesis is maybe it was seeing an unexploited cornucopia of food on land – millipedes, centipedes, spiders and more – that drove evolution to come up with limbs from fins.”
Invertebrates came on to land 50 million years before our vertebrate ancestors arrived.
The enlargement of eyes is significant.
By just popping them above the water line the fish could see 70 times farther in air than in water.
With the tripling of eye size the animal’s visually monitored space increased a millionfold.
This happened millions of years before fully terrestrial animals existed.
Prof Lars Schmitz, of the W.M. keck Science Department in Claremont, California, said: “Bigger eyes are almost worthless in water because vision is largely limited to what’s directly in front of the animal.
“But larger eye size is very valuable when viewing through air. In evolution, it often comes down to a trade-off.
“Is it worth the metabolic toll to enlarge your eyes?
“What’s the point? Here we think the point was to be able to search out prey on land.”
Larger eyes were consequently selected for, whereas the study shows that in water, larger eyes led to negligible increases in visual range.
In fact, one animal group that arose after animals came onto land went back to full-time life underwater.
Their eyes went back to the smaller eye size normally seen in fish, the study published in Proceedings of the National Academy of Sciences found.
The massive increase in visual capability enabled by vision in air likely allowed early-limbed animals to evolve more complex cognition.
These animals were no longer forced to react with split-second speed as was required by life in the vision-limiting water.
Eventually evolution led to the human capability of prospective cognition – the power to weigh options for the future and to choose strategically, said the researchers.
Prof MacIver and Prof Schmitz studied 59 fossil specimens spanning the time before, during and after the water-to-land transition.
Their computer simulations of the animals’ visual environments – such as clear or murky water in the daytime or above water in the daytime and the night-time – show the benefit of increased eye size would be realised when an animal is seeing through air, not water.
The researchers measured the size of each fossil’s orbits, or eye sockets, and head length. From that, they determined the size of the eyes and the size of the animal itself.
They found before the water-to-land transition the average orbit size was 13 millimeters and around the time of it this had increased to 36 millimeters.
Prof MacIver said: “The tripling of orbit size took 12 million years. This is the timescale of evolution, which boggles our mind.”
Rather than limbs it was eyes that brought our ancestors to land thanks to long-range vision that drew them to an abundance of food on land, said the researchers.