I can think of more than a handful of reasons I can relate with this sentiment. One in particular is simply how convenient things aren't. Nature has had to constantly come up with solutions to a lot of problems that never needed to be there in the first place. One of the four most major problems, we call Gravity.

The story goes that gravity forced nature to sustain itself in water to counteract its effects, but when that got boring, land became inevitable and yet more solutions were needed such as rigid bones to hold us up under all that weight. To make things easier, we became Bilateral and developed lungs for air.

But this assessment of history isn't completely true. So let's dig in and have a proper look at how limbs came about, and how we started to prepare ourselves for land.

The Humerus

Back in 1993, a humerus fossil - the bone from the upper arm - was found in a clump of sandstone. Thinking little of it, paleontologists just kinda dumped it with the collection and left it at that for a decade. Then, in 2004, some bored individual decided to extract it from its prison and see what secrets it had locked away with it.

It turns out that the arm bone was from around 365 million years ago. This makes it one of the earliest examples of tetrapods to ever exist and its age comes with a lot of implications to the story so far.

Tetrapoda

This superclass contains about 32,000 species that branched off from lungfish and other unimportant life presumably around the time this arm bone came about. This group includes reptiles, amphibians, birds and mammals, all of which have four (tetra) limbs (poda), and made the transition from water to land. But the question of why we all have four limbs was overturned with the examination of this ancient humerus.

Originally, the idea was that we grew limbs with bones to help us pull our clunky bodies from the sea to land, but the above arm bone comes from way before we made that move, so the narrative had to change. In fact, bones themselves were likely formed initially for protection in the sea, but also for hunting advantages in the form of the jaw bones in the skull that allowed bigger, scarier mouths. None of this had anything to do with moving on to land.

Our general perception of a clumsy fish walking out of a vast, open ocean and onto some muddy landscape is an important misinterpretation we should probably unlearn anyway. It's so much more complex and fascinating than that. And besides, most modern day marine life tends to be coastal or in other shallow water where minerals and plants are rich and easy-access; rivers, streams, estuaries and so on. This wouldn't have been any different way back when.

Over time, those plants grew thicker and more intricate, their biomass remains became muddier and cloggier. As a result, it would have been an evolutionary advantage to have weight-bearing fins, supported by their newly developed bones to help them trudge along shallow waters and shorelines, with those fins eventually specializing into limbs. It's also thought that these fins could have allowed fish to grip on to plants to anchor them in place while waiting in ambush while water pushed them around.

_{Source CC BY 2.5}

The idea of actually moving fully onto land need never have been considered for this to happen, and this is the secret that 365 million year old limb revealed to us. Other stages of evolution demonstrated this same motive from even further back, with the Kenichthys, Gogonasus and Panderichthys being examples of fishy-type Tetrapodomorpha who developed fins that aided in movement through plant-dense waters up to 390 million years ago.

Adding to these modifications, the shoulders changed position to become sideways facing, allowing a right-angled trunk of an arm, kind of like what we see in crocodiles, making them more useful at bearing the weight of clunky bodies than paddling around.

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And then...

With plants having dominated the oceans, shores, and land long before animals, a re-balancing of oxygen content was inevitable, with less dissolved in the oceans, and more being excreted by plants into the air.

It was only natural for the balance of life to shimmy towards the oxygen-rich land where they also get to escape scary, bony-jawed predatory fish and brutally fast cartilaginous sharks, with the further benefit of filling a new niche, with the previous ocean market already saturated. Lotsa good stuff up there.

So, fins became limbs, and thanks to our bilateral nature, we ended up with four of them. Lo' and behold, we have tetrapods. But tetrapods are not indicative of being a landlubber. For that, we need to look at nostrils. More on that in the next post. We're not hitting land just yet. Patience.

Tiktaalik

The arm bone is not the only evidence that backs up the idea that bony limbs evolved before we touched land. The pelvic bones of the lobe-finned (the type of fin we associate with modern fish) Tiktaalik, a candidate for a transitional specimen into tetrapoda, shows us that back legs and pelvic-powered motion came before land, too.

_{Note: depiction's eyes too large. Public Domain}

In 2014, the pelvic girdle of the animal was discovered, showing it had a particularly strong build, indicating its ability and tendency to push itself along shallow water and mud flats. It's theorized that, like the mudskipper fish of today, the tiktaalik would occasionally venture out onto the muddy land, perhaps to cross to various pools of water or just to be adventurous.

But it's not all about the legs. There were other, perhaps even more pressing obstacles that needed to be dealt with before making a comfortable home on land:

The Buccal Cavity

Also known as a mouth, the buccal cavity is another indicator that tetrapods developed prior to landfall by looking at how food was eaten. Looking at modern fish, we see an animal that can easily catch prey, even on land in some cases, but they still need to drag it back into the water to actually consume. This is not just to drown its prey, but to take advantage of the hydrodynamic forces that occur when changing the ambient pressure within the mouth.

Next time you have a bath, dunk your head under the water and then force your mouth open. If you're not one of those freaks with one too many face piercings, you should witness water being sucked into your mouth. The fact that a living organism has roughly the same density as water allows the fish to swallow like this.

But it would be far more difficult to do this on land where the density of air is far less. Similarly, early tetrapods had not yet evolved the skull and jaw systems that allowed them to actually swallow properly without the aid of water.

This means that even if tetrapods of the time could get on land, as they occasionally did, they wouldn't stay there, or they'd starve to death.

They wouldn't suffocate, though. But that is for another day.

Next time we'll finally see how we finally made the step from temporary to permanent landfall, and never looked back.

...Well, except whales, dolphins, otters, walruses, seals, spiders... whatever, you get the idea.

References: _{The Early Evolution of the Tetrapod Humerus | Tetrapodomorpha | Evolution of Tetrapods | Pelvic girdle and fin of Tiktaalik roseae | Vertebrate Land Invasions–Past, Present, and Future: An Introduction to the Symposium}

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