We know that a significant event in evolution occurred when animals climbed out of the water to live on the land. Mammals started out as tetrapods, or four-legged animals. Most mammals are still traveling on four legs. But a bipedal species has evolutionary advantages. It has a better view of approaching predators, can wade into deeper water for fish, and reach up higher to snag fruit from the trees. In order to stay upright (and keep moving), the hip bones in humans needed to be changed from a locomotive device (helping to use the back legs to travel) to a load-bearing one (keep the top half on top). Walking upright caused the brains of human beings (in the area of the frontal lobe) to improve, making humans became the best kind of mammal around.
Now it seems that strong hip bones got tetrapods out of the water about 395 million years ago. Hip bones built up in certain fish through the development of the ilium, the butterfly-shaped bone in the pelvis connected to the spine. The ilium is one of the three bones forming the hips.
Muscles in present day fish, such as the lungfish, may have also aided in the transition from sea to land. In addition, lungfish also have “sitting bones” in the pubis. In our ancestors, these bones were gradually stretched from the pubis to form the hips.
This is not to suggest that fish or amphibians were the first residents of the land. A recent study of fossils of various frondlike and sacklike organisms that were thought to live at the bottom of the ocean may have been the earliest organisms to reside on land. The transition from sea to land may have occurred more than many millions of years ago.
The move from land to sea was also preceded by the development of “lobe-fins”—fins resembling paddles—and the concomitant exchange of lungs for fins. It is estimated that all this happened about 400 million years ago.
Assuming that tetrapods were the first ones out of the water, the question remains: why would they want to move?
One theory is that drought reduced the size of oceans, forcing animals to go ashore. The ones that could breathe are the ones that survived.
Other scientist dispute that the periods of drought match pioneering to land. They suggest that the high population of competitors and predators in the water may have made it safer to spawn ashore, where the threats and the competition were less severe. Or the shallow areas in the swamps of the Devonian period—about 400 to 350 million years ago—may have been low on oxygen content, forcing tetrapods to find breathing room on land.
Environmental changes have always spurred creatures to seek new habitats. In recent times, the destruction of habitats by humans—cutting down rain forests, introducing greater and greater levels of pollution, the acceleration of global warming—cause thousands of species to be without a home, which forces them to migrate. But the number of alternative niches is dwindling. And those that cannot find a place to live will face extinction.
Our habit of disruption of the earth for our own purposes resembles the species in the ocean that could not relocate to the land millions of years ago, and disappeared forever. We need to consider the consequences for the future.
Written by Tom Ukinski