Some animals get all the glory. That's doubly true when it comes to one of nature's most spectacular feats: flying.
We're all familiar with the birds and the bees – and other insects. Bats are also well-known for their aerobatics. And lovers of spectacular prehistoric beasts will know about the pterosaurs - extinct flying reptiles, some of which had the 10m wingspan of a light aircraft.
But the annals of flight carry the tales of plenty more pioneering species. From prehistoric flying fish and jet-propelled squid to flying trees and truly weird gliding reptiles, these are the aeronauts you never knew existed.
There's a good reason why the story of flight focuses on birds, bats, pterosaurs and insects. They are the only groups ever to have evolved sustained powered flight – although humans have found a few ways to do it too.
This famous four are far from being the only flight pioneers
Insects were the first group to achieve the feat: they developed wings at least 320 million years ago. Pterosaurs were the first vertebrates - animals, like us, with a backbone - to evolve powered flight, about 230 million years ago. Even more recently, bats were the first mammals to truly fly, beginning about 50 million years ago.
But this famous four are far from being the only flight pioneers. At least two groups of backboned animals beat the pterosaurs into the air by millions of years.
A good 10 million years before the pterosaurs began swooping between trees, fish had begun gliding over the ocean waves.
Most people have heard of flying fish. These sea-dwellers have long wing-like fins, which allow them to coast through the air for tens of metres if they catch a favourable breeze.
All of today's flying fish are closely related to one another, and belong to a family called the exocoetids. They aren't much older than bats, having evolved perhaps 65 million years ago, and we don't know much about how they did so, says Guang-Hui Xu at the Chinese Academy of Sciences in Beijing.
But an earlier, now extinct group of fish called the thoracopterids turned their fins into wings at least 240 million years ago. In January 2015, Xu and his colleagues explained how they did so, with the help of some new primitive thoracopterid fossils from China.
"Over-water gliding adaptations were gradual in nature," says Xu. The process began in the skull. The new fossils, called Wushaichthys, had a broad and flat skull roof typical of thoracopterid fossils. This possibly helped the fish swim and feed just below the ocean's surface.
The second step was the crucial one. Some thoracopterid fossils from around the same time added a specialised tail fin, with a lower fork much longer than the upper fork. It's this asymmetrical fin that helped thoracopterids gain enough power when swimming to jump out of the water, making it the key characteristic for identifying flying fish, says Xu.
Gliding over land is very energy-efficient, whereas gliding over the ocean isn't
Only later did the thoracopterids evolve the wing-like fins that allowed them to make best use of their jumps, by gliding. Finally, the thoracopterids lost their body scales, perhaps because doing so made it easier to wiggle during glides to improve flight efficiency.
The ancient thoracopterid fossils give us a good idea how today's flying fish evolved. Modern exocoetid flying fish also have broad skulls, asymmetric tail fins and long wing-like fins, and probably evolved in a similar way, says Xu.
It might seem odd that fish evolved the same flying ability twice, but perhaps the bigger surprise is that they didn't do it more often. After all, a host of land animals have evolved the ability to glide. Xu says it's because gliding over land is very energy-efficient, whereas gliding over the ocean isn't. "The gliding of flying fishes is energetically very expensive," he says.
Thoracopterid fish are among the earliest backboned animals to take to the skies, but they probably weren't the first. That title currently rests with a 40cm reptile that lived about 255 million years ago. "Coelurosauravus is still the oldest gliding vertebrate," says Hans-Dieter Sues at the Smithsonian Institution in Washington DC.
Coelurosauravusis the very quintessence of oddness. When the first specimen was being extracted from rock, researchers found an array of long rod-like bones near the rib cage. They assumed these rods were fin rays of a larger marine fish that had, by chance, come to rest on the dead reptile. So they removed most of them.
I know of nothing similar to the rods supporting the gliding membrane in Coelurosauravus
But in 1997, Sues and his colleagues examined some more Coelurosauravus fossils, and showed that this was a mistake. The rod-like bones actually belonged to the reptile, but they weren't part of its normal skeleton. Instead, they had grown in the animal's skin, effectively forming a second skeleton.
Some other animals grow bones in their skin, usually to toughen it up. These "osteoderms" are what gives crocodile skin its scaly appearance. But Coelurosauravus's osteoderms are different. Instead of serving a defensive role, they supported a membrane that could expand into a large gliding wing. "I know of nothing similar to the rods supporting the gliding membrane in Coelurosauravus," says Sues.
In other words, not only was Coelurosauravus the pioneer of backboned flight, the strange wings it evolved are like nothing else that has evolved before or since. It was a true maverick, and ought to be a household name. It only gets less attention because it was a glider rather than an active flier, says Sues.
But then, even active fliers sometimes get forgotten. For instance, squid can fly.
Yes, really. Squid occasionally join flying fish above the waves. They just tend to do so under cover of darkness. "That might explain why fishermen never seem to see squid flying," says Ronald O'Dor at Dalhousie University in Halifax, Nova Scotia.
The squid accelerated through the air
This also makes studying the behaviour very difficult, even though scientific accounts of flying squid go back to the late nineteenth century. But it shouldn't be that surprising. Squid use a natural jet-propulsion system to move underwater, and it ought to be powerful enough to shoot them up and out of the water. Then the squid's fins and tentacles can double as wings.
What's more, that jet-propulsion system may continue to work even once the squid is airborne. When black-and-white footage of a 1.2m Humboldt squid flying was studied in 1970, the analysis showed that the animal accelerated through the air. This means squid flight can be both active and powered - just as in birds, bats, pterosaurs and insects - earning the tentacled invertebrates a special place in the annals of flight.
There is a big difference between squid and other active fliers, though. A squid can only keep it up for a few seconds while it is jetting water out of its body cavity. Squid are incapable of the sustained powered flight seen in the four famous groups of flying animals.
Nevertheless, flying clearly has benefits for the squid, says O'Dor. Flight is energetically expensive for flying fish, but it's not for squid. "It's clearly a more energy efficient mode of transport than swimming," says O'Dor.
Squid have such muscular bodies that breaking the surface of the water is relatively easy. They can rocket to 6m above the surface, before gliding effortlessly for tens of metres."Flying fish are not nearly as good at flying as squid," says O'Dor. He has a hunch that some squid may routinely fly by night during migrations, to cover ground more efficiently than by swimming alone.
Modern squids probably began flying just a few million years ago
But even though flying is so beneficial, squid probably haven't been doing it long, according to Dirk Fuchs at Hokkaido University in Sapporo, Japan and Royal Mapes of Ohio University. They study fossil squid and other cephalopods, and have found nothing as muscular as today's squid until relatively recently in geological time.
The muscle of a squid mantle is ring-shaped – that's why calamari comes in rings - giving it strength. There were squid-like animals called belemnites from 200 to 65 million years ago, but they had weaker, U-shaped muscles in their mantle. "A calamari steak rather than rings," says Fuchs. So while belemnites looked a lot like squid, they almost certainly couldn't fly. Modern squids probably began flying just a few million years ago, says Fuchs.
So what were the first organisms to fly? It's hard to be sure, but it may not have been an animal. It might have been a plant.
Plants may have developed wings at the same time as insects. Wings on their seeds, that is. Winged seeds have been discovered in rocks that are 360 to 370 million years old, while fossils suggest that insect wings evolved around 400 million years ago. But the fossil record of land-dwelling organisms is so patchy this far back in time that it's difficult to say for sure which wings came first.
By 270 million years ago, conifer trees had developed a new form of flight that isn't seen in insects. Their winged seeds evolved to rotate, helicopter-style, as they fall. Today's winged conifer seeds still have this ability – and probably inspired humans to develop helicopter designs.
The seeds can fly like a helicopter because they almost always have just one long wing, to counterbalance the weight of the seed body. The helicopter spin slows the seed's descent through the air, so it can "fly" a fair distance from the parent tree before it finally touches down.
Curiously, though, some of the earliest helicoptering seeds didn't have just one wing – they had two. In 2014, Cindy Looy at the University of California, Berkeley examined the fossil seeds of the oldest known helicoptering conifer, called Manifera. At 270 million years old, they predate all other known examples by at least 10 million years.
Unlike later seeds, or those found today, most of the seeds had a small second wing on the opposite side of the seed body from the main wing. About one-tenth of the seeds went further even than this: they had two symmetrical wings, one on each side. That was strange, she says, because nowadays double-winged seeds "are really rare". Looy is now exploring how well the double-winged seeds can fly.
Regardless of whether plants or insects were the first to develop wings, neither of them was the first group of organisms to take to the skies. The first airborne life forms were almost certainly bacteria, says Kostas Konstantinidis of the Georgia Institute of Technology in Atlanta. In 2013 he examined samples from 8-15km up in the air, and found hordes of bacteria - accounting for 20% of all the particles between 0.25 and 1 micrometer in size.
High-flying bacteria have been influencing Earth's climate for billions of years
Konstantinidis says he is "100% sure" that microbes, and especially bacteria, have been traveling through the atmosphere for a very long time. "Just consider that the first microbes on Earth appeared around 3 billion years ago," he says. Animals and plants didn't really get going until 500 million years ago.
Bacteria are not active fliers, of course. They haven't even developed wings to help them glide. Instead, they are probably swept up into the air on the breeze simply because they are so small. But Konstantinidis's research suggests that these atmospheric bacteria are anything but irrelevant.
He says they may help clouds to form. Clouds are made of billions of water droplets, each of which condenses around a tiny nucleus of material such as a dust mote. Bacteria are the right size, so water droplets may form around them, too.
If Konstantinidis is right, high-flying bacteria have been influencing Earth's climate for billions of years. No other flying species can claim that.