fabian s. klinkeThe Hardest Problem Evolution Ever Solved
Source: The Hardest Problem Evolution Ever Solved, Hank Green, 20:36, uploaded 2025-07-09, playlist index 10.
Hank Green begins with a complaint about dry land. Fish have buoyancy, water keeps their eggs from drying out, and the surrounding temperature changes less violently. On land, gravity holds the body up, the sun reaches it directly, and water leaves through every exposed surface. A fish moving onto land enters a different physical world with a single body that has to solve several problems at once.
The awkward climb from water
The transition takes place over a long interval. Vertebrates existed for tens of millions of years before any of them left the water, and the first animals that did so stayed close to the ocean for millions of years afterwards. Early land vertebrates had lungs and limbs, yet they also carried gills, weak rib cages, and limbs too flimsy to support their bodies for long on dry ground. Green describes them as aquatic animals with a few pieces of land equipment attached.
That order matters. Evolution does not wait until a complete land animal appears and then send it out of the water. Small advantages accumulate among organisms that die often enough for those advantages to spread. Some lineages try a new environment and disappear. A lasting transition requires several survival problems to resolve in parallel, with each solution bringing its own costs.
Green starts with limbs because the fossil record makes their history unusually visible. Lobe-finned fish have thick, fleshy fins with muscles and bones inside them. Their bones follow a pattern recognisable in our own arms, including the radius and ulna and the small bones that lead towards digits. Fossils such as Tiktaalik show related structures moving through time into fish-like animals that could push themselves along the ground.
Ray-finned fish have taken smaller steps in the same direction. Mudskippers prop themselves up on their fins, push with their tails, and spend long periods out of the water. Their fins lack the strong internal bones that made the lobe-finned route useful, so their excursions remain tied to wet ground. They solve other problems instead, including some of the eye and skin problems that appear later in the story.
The old fin has to become a weight-bearing limb on both the front and back of the body. Its bones need to thicken, and the muscles need new attachment points. Gravity gives the change a reason to exist because buoyancy has stopped doing the work. Green places this adaptation among the hard parts of the transition, then turns to a system that seems harder still: vision.
Eyes built for water
Fish eyes evolved in water, where light bends as it enters the eye in a way that suits the fish’s lens. The same lens performs poorly in air. The familiar blur that appears when a person opens their eyes underwater comes from the changed path of light at the water and cornea, rather than from pressure on the eyeball. A fish that leaves the water therefore needs a different optical arrangement.
The lens contains proteins called alpha-crystallins. Green presents these as an example of neofunctionalisation, the adaptation of an existing structure or protein for a new job. Alpha-crystallins first served as protection for other proteins under heat or ultraviolet stress. When arranged in the lens, their transparency lets them take part in focusing light.
Changing the lens itself would have been one possible route. Early land vertebrates instead gained a curved cornea that compensates for the difference between water and air. Their eyes still carry a lens suited to the aquatic environment, while the shape around it corrects the optical error. Focusing also changes. Fish move the whole lens forwards and backwards. Land vertebrates squeeze and stretch it with small muscles, which amounts to a new focusing system built around the old part.
Air then adds several smaller problems. A working eye needs a wet surface, so land animals need eyelids, tear glands, and drainage paths. Pigment protects the retina from direct solar radiation. Mudskippers reach a similar practical result through a different structure: the eye retracts into the body and a flap of skin covers it. Similar pressures can produce different anatomical solutions because evolution works with whatever material a lineage already has.
The water that has to stay inside
Green’s hardest problem arrives when he asks what happens to the water inside a living body. Every cell remains a small watery environment, while the air outside draws water away. Moving onto land therefore means carrying an internal version of the ocean and reducing the leaks. The ancestors of land vertebrates had skin that allowed water to move through it. They needed a flexible surface that could hold water without becoming a rigid shell.
The solution is keratinised skin. Keratin is a tough fibrous protein packed into the outer layer of skin, where the cells have already died and sealed themselves. With collagen and related structures, it forms a flexible barrier that limits evaporation and resists damage. Green calls this the most important adaptation in the transition because it makes life away from permanent water possible.
The origin of that barrier is another case of neofunctionalisation. Fish already contain keratin, though fish scales are not made from it in the way human hair and nails are. Inside fish cells, keratin forms long strands that help maintain their shape and mechanical strength. Green’s account has evolution increase the amount of this existing material in skin cells. The cells then release their water, fill with keratin, and bind together with collagen-related proteins. An internal support material becomes an external waterproofing layer.
Once that surface exists, evolution can use it for many other structures. Scales, claws, beaks, feathers, horns, hair, and the pads on an animal’s feet all draw on keratinised skin. Frogs show the compromise at the edge of the solution. Their skin contains enough keratin to slow water loss, while remaining thin enough for some gas exchange. They stay dependent on a damp environment because the barrier is useful without being complete.
Green keeps this section deliberately partial. He leaves out hearing, eggs, sex, heat, and ultraviolet protection, along with many physical, genetic, and biochemical changes that the move onto land required. The list of adaptations gives an account of the problems that make the transition legible. It does not provide a complete history of terrestrial vertebrates.
Lungs before land
Lungs seem like the obvious answer to the question of how fish began breathing air. Green’s research gives them a different place in the sequence. Lungs appear in the ancestor of bony fish as early as about 420 million years ago, long before vertebrates established themselves on land.
Water holds less accessible oxygen than air, and oxygen takes time to diffuse through it. Shallow, muddy water can lose its oxygen altogether. A pouch connected to the digestive tract lets a fish gulp air and absorb some of its oxygen through a blood-rich surface. That small increase can matter a great deal in stagnant water, so air breathing becomes a useful adaptation even when the fish remains aquatic.
The lung itself is a transformed part of the gut. Green uses this to explain why the mouth still serves both eating and breathing. He then reverses the expected relationship between lungs and swim bladders. It is tempting to imagine that an air-filled swim bladder came first and later became a lung. His account follows the opposite direction: swim bladders are lungs that became buoyancy organs.
Fish that lost the original lungs could evolve air breathing again through other structures. Catfish use a pouch connected to the gut. Mudskippers trap air around the gills and use muscles that keep those gills from collapsing. Bettas use a high-surface-area chamber connected to the gill region. Green says air breathing has evolved dozens of times, which explains why the solutions vary so much. The lung lineage that humans use is one old route among several.
That history lowers the status of lungs in the land transition. They are necessary for animals that breathe with lungs, yet they were already available to bony fish and repeatedly useful in water. Limbs solve the weight problem, eyes solve a new optical problem, and keratinised skin solves the problem of keeping a body wet. Lungs arrive early because the selective pressure for them already exists in the water.
The fish that remains
The final move concerns classification. The phrase “fish don’t exist” works as a science joke because humans, frogs, birds, and dinosaurs belong inside the evolutionary history of fish. A taxonomic group that contains every fish ancestor must also contain the land vertebrates that descend from them. A physiological definition produces a different boundary, since “fish” can mean an aquatic vertebrate and whales fit that description more easily than most people allow.
Green stays with ancestry. Humans are more closely related to bass than bass are to sharks, so humans and bass share a more recent common ancestor within the bony-fish lineage. He uses that relation to describe us as strange fish covered in keratin and breathing through lungs. The phrase works because each part of the earlier account remains visible in the body.
Our cells still hold the water that life needs. Our eyes carry a lens whose optical history belongs to water. Our lungs branch from the digestive system. Our skin uses a protein that first helped hold a cell together. “You are a fish and you never really left the water” is Green’s conclusion, and its force comes from the mechanisms that precede it. The ocean remains inside the body as physiology, chemistry, and ancestry.
Limits of the account
This note follows Green’s 20-minute synthesis without independently checking each claim in the references below. The description credits Deboki Chakravarti with fact checking and lists eleven scientific sources, yet Green also says that his adaptation list is incomplete. The dates, evolutionary relationships, and examples stay within the source’s explanatory account. Questions about the exact origin of particular structures require the individual papers and datasets.