From Water to Land - the Vertebrate Land Invasion

Until a few hundred million years ago, our vertebrate ancestors were swimming (perhaps happily!) under water. They were well adapted to its environment. So why and how did they migrate and subsequently adapt to the terrestrial environment? As Adams mentions in his book The Hitchhiker's Guide to the Galaxy, not many are convinced that this was a wise decision!

Many were increasingly of the opinion that they’d all made a big mistake in coming down from the trees in the first place. And some said that even the trees had been a bad move, and that no one should ever have left the oceans.

Evolutionary biologists predict that this transition occurred at shallow waters, swamps or mangroves, where they were in contact with both water and land. Under water, the organisms were at competition for resources and at a risk of predation. Perhaps the yet unconquered by vertebrates land proved to be a rich niche with plenty of resources and little-to-no competition.

Researchers speculate that the driving force for movement to land was vision. The physics of vision is much different under water than in air. Under water, vision is poor owing to two reasons: the refractive index associated with water and animal tissues are largely similar; and the rapid attenuation of light over a distance at water. Consider the human eye, light on reaching the cornea, refracts due to the sharp change in the refractive index from air to a tissue, and this helps focusing the image on the retina. However, we see things blurred because of the difference in refractive index. In case of the attenuation of light under water, imagine holding a solid surface in the path of light. A part of the light gets reflected, scattered, absorbed and only a part gets transmitted. The same phenomenon happens in a water body, i.e. the organisms can perceive only a small part of light. This may have limited their visual range. However, at land owing to the absence of these two drawbacks, the visual range and acuity increased. Further, researchers have discovered from fossils that the eye socket and eyeball size increased in this transition. Larger eyes meant larger pupil size, which would allow more light to focus on the retina. Another adaptation that happened was the movement of eye from the side of the head to the top so that the organisms can see over water, (like the eyes in crocodiles!). So, to conclude, the organisms could see farther and with better clarity in land than water. This is a major advantage not only to spot prey and predators it is also suggested that this long-range vision helped the development of complex planning as seeing farther enabled organisms to plan their time and manner of attacking prey and similarly evade predators. This ultimately might have led to our ability to think Prospectively, Contemporaneously and Retrospectively.

But for an organism to leave water, there were several other challenges – all regulated by the surrounding fluid change – from water to air, and the major concern being air itself. Gills are adept in drawing the dissolved oxygen in water, but in air they were ill-equipped. Contrary to popular belief, lungs did not evolve from gills, it was the fishes’ digestive system which adapted to form lungs. The first Tetrapods (organisms that have four limbs – today includes amphibians, reptiles, mammals and aves) breathed air by swallowing it and absorbing oxygen in the gut. Over time, a special pocket formed for this purpose- the lungs.

Next, to prey on land, the organisms must move on land, which is a major challenge as under water, the effect of gravitational force is comparatively less than on land When an organism is under water, it experiences an upward ‘buoyancy force’ equivalent to the weight of the fluid that would fill the volume occupied by the organism, which acts against the gravitational force, thus leading the organisms to experience lower gravitational force. As a result, only the organisms that were better adapted to survive the higher gravitational force on land would survive. This adaptation happened in the form of improved musculoskeletal system, which would be able to bear the weight of the organism. Fossils of fishes having muscular fins were discovered and it was proven that this was among the major adaptations that happened in this transition.

Another problem with air is that it tends to dry up everything. The need to maintain water balance by preventing evaporation is important – not just for maintaining body homeostasis but also for reproduction. At first, the Tetrapodsstayed in moist environments like the amphibians to stay hydrated. However, animals which developed a water-proof layer (the skin) to avoid dehydration were able to survive better even in dry regions. Similarly, recent studies show that dehydration changes the eggs’ composition and yolk immune function and hence, like amphibians, the early Tetrapods, laid their eggs under water to prevent dehydration. However, evolution selected for organisms which were able to encase their eggs in layers to prevent dehydration – today what we call the ‘amniotic membrane’ which gave rise to the clade amniotes.

Thus, the evolution of terrestrial vertebrates brought in several issues despite which, for organisms which are mostly unable to survive a change in the salinity (fresh water to marine), genetic changes and natural selection equipped our ancestors in surviving the migration from land to water, and led to the diversity in life forms we see today. I would conclude by stating that, the transition from water to land was not a simple process, having taken millions of years. It is disheartening that the sixth wave of mass extinction happening due to our activities (hence termed Anthropocene extinction), is in turn leading to the loss of many of these life forms hinting a future wherein we would have only humans and human domesticated animals. It is high time we put our ability to think Prospectively, Contemporaneously and Retrospectively to better use for the sustenance of biodiversity.