From the very beginning, the earth was void and without any form. Darkness prevailed upon the face of the deep until a giant cloud of gas and dust collapsed to form our solar system. Around 4.5 billion years ago the molten state of the earth began to cool down. The powerful collisions with comets and asteroids brought the fluid of life that is Water. With the advent of water, clouds and oceans began to take shape. It was not until a billion years later that the first life was brought forth. From here started the transition of life underwater and slowly moving from water to land. Though it was a small step for fishes, it gave a giant leap for animal kind!
Primarily, life on Earth began in the water. So when the first animals moved out of the sea, they had to swap their fins for limbs and gills for lungs in order to adapt better to their new terrestrial environment. Looking at modern fish species it does not seem so hard to ideate the slow adaptation of life on from water to land. More than 350 million years ago our distant ancestors of fish showed a transition of life from water to land. It all began with single-celled organisms fusing into multicellular. This made their body diversified and radiated, exploding into a group of invertebrates.
Yet all this abundance and life was restricted to the seas whereas the vast bountiful land was standing unused till then. Around 530 million years back, evidence showed that centipede-like animals began to explore the world above water. Plants played a crucial role in creating a land rich in food and resources as they colonized the bare earth. This happened somewhat around 430 million years ago. Another 30 million years back prehistoric fish crawled out of the water and began the evolutionary lineage we sit atop today. Our ancestors braved a brand new world above the waves.
This is where we came from!
The four-limbed vertebrates that first braved life on land were the Tetrapods. They were the direct descendants of ancient fish. The tetrapods’ move to land has long been one of the great evolutionary puzzles as a number of fish exhibit traits that are not unlike those of the first tetrapods. Once they moved from water to land, these four-limbed vertebrates branched into an impressive range of animals which include amphibians, reptiles, dinosaurs, birds, and mammals. The fossil record shows that as species originated to fill particular ecological niches, a few of the tetrapods lost their limbs (snakes), some of them turned arms into wings (bats, birds, and pterosaurs) and some decided to disarray dry land and headed back to the sea (including whales, seals, and some marine reptiles).
Studies suggest that a shift to lungs and limbs doesn’t tell the full story of these creatures’ transformation. As they came out of the sea, they gained something perhaps more precious than oxygenated air: Information. The ability to see is much more in the air than underwater. So the increased visual range provided as an “informational zip line” that alerted the ancient animals to bountiful food sources near the shore. Therefore, researchers suggest that the ancient creatures that first crawled out of the sea onto the land may have been lured by the informational benefit that comes from seeing through the air.
Although tetrapods achieved such a feat, they faced many barriers that lay between their life under the sea and the land above that awaited them.
Living in the air instead of water is packed with many difficulties. Locomotion is one of the problems. It is not a major problem though as evolution in a number of lineages has shown. While mudskippers and catfish seem to walk with ease, the same was not in the case of our ancestors. Some of the earliest tetrapods like Ichthyostega were quite cumbersome on land and spent most of their time in the comfort of water. These first tetrapods are traced from an ancient lineage of fishes called the Sarcopterygii or Lobe-Finned Fish. As the name signifies, these animals have paddle-like fins instead of the flimsy rays of most modern-day fish species. These lobe fins, covered with flesh, were ripe for adapting into limbs.
Early tetrapods faced yet another problem. They had to develop more than a new way to walk as their entire skeletons had to change to support more weight (water supports mass in a way that air simply doesn’t). Each vertebra had to become more powerful and stronger for support. Ribs and vertebrae changed shape accordingly and evolved for extra support to better distribute weight. Skulls got disconnected and necks evolved to allow better mobility of the head and to absorb the shock of walking. Bones were shifted, streamlining the limbs and creating the five-digit pattern that is still reflected in our own hands and feet. Joints were rotated forward to allow movement for four-legged crawling. This was how a body plan developed in 30 million years or more which became fit for walking on land.
Another problem with air is that it tends to make things dry. You may have heard that our bodies are 98% of water. As well evolved land organisms we have highly developed structures that ensure that all the water doesn’t simply evaporate. The early tetrapods required this feature to develop on its own. Initially, many tetrapods usually stuck to moist habitats in order to avoid water loss. But eventually, in order to conquer drylands and deserts, animals had to find other ways to prevent themselves from drying out. It is even thought that many of the early tetrapods began experimenting with different ways to waterproof their skin. Even more important was the problem of dry eggs. Amphibians could solve the dryness issue by laying their eggs in water, but the tetrapods which conquered land didn’t have that luxury.
The solution to land’s dry nature was to protect the eggs in a number of membrane layers, which is now known as an amniote egg. Even human babies grow in an amniotic sac that surrounds the fetus. This adaptation allowed animals to overcome the problem of watery habitats and distinguished the major lineage of tetrapods including reptiles, birds, and mammals, from amphibians.
Such key adaptations by tetrapod skeletons and anatomy allowed them to conquer the world above the waves. Without their evolutionary ingenuity, a diverse set of animals, including all mammals, would not be possible today. Yet still, we barely understand the ecological settings that drove these early animals from water to land. Did dry land offer an endless bounty of food to not be passed up? Perhaps, but there is evidence that our ancestors braved the dry world very early, even before most terrestrial plants or insects, so it is possible that the earth was barren. Were the animals trying to escape competition and predation in the deep? Or was land important for some yet undetermined reason? We may never know. But as we reflect upon our beginnings, we have to reward credit to the daring animals that began the diverse evolutionary lineage we are a part of. While we may never understand why they left the water, we are thankful that they did so since it bought transition of life from water to land!