From Sea to Land and back to Sea!

The migration of animals from sea to land and then subsequently from land environments back into the sea is a fascinating journey. It is widely considered to be one of the most important steps in the evolutionary history of life on Earth. The formation of the ozone layer, high in the Earth's atmosphere about 600 million years ago provided organisms with a vital shield from harmful ultraviolet radiation. Photosynthesis, which had  been occurring for many millions of years prior to this, facilitat

ed this development by oxygenating  the atmosphere. This increase in oxygen levels was also necessary for aquatic animals looking to venture  onto land for the first time. The early Devonian was a major milestone for terrestrialisation marked by the appearance of lobe-finned fish or SARCOPTERYGIANS. This group of fish have characteristically powerful and muscular fins - features which would prove invaluable in their bid to invade the land. Plants were already well-established on land by this tim

e and they diversified and  evolved rapidly during the Devonian leading to the emergence of the first forest environments. This effectively established a new habitat - a feasting ground for land animals to exploit. A group of  vertebrate animals with four limbs known as TETRAPODS had been evolving in aquatic environments, and were seemingly well-suited, in fact almost pre-adapted to make this important transition  onto land. Land also provided the benefits of basking, where tetrapods settled in

mud-banks to enhance digestion, growth and maturation rates by heating their bodies whilst increasing reproductive  success by securing safe zones for laying eggs or predation opportunities were limited compared to the treacherous rivers, lakes and seas. In the middle Devonian, evolution saw the emergence of the  peculiar TIKTAALIK an intermediate creature between fish and tetrapod possessing powerful flipper-like  lobe fins for swimming, an indistinguishable neck for grasping prey, broad overla

pping ribs for  support and was equipped with girdled bones in the shoulder. Not long after came along ICHTHYOSTEGA capable of stabilising Itself by hind limbs, rather than lobe fins, boasting a rigid rib-cage with a large  skeleton Ichthyostega supported its chest by overlapping ribs attached to the vertebrae and possessed seven  digits in the limbs which resembled a foot allowing for mud skipper-like locomotion to occur, helping it traverse in shallow waters via punting until eventually decidi

ng to venture out to conquer land. After the tetrapod amphibians had fully conquered the land, a new land animal evolved during the  Carboniferous: REPTILES - creatures with watertight skin that lay eggs with tough outer shells. These adaptations meant they no longer had to return to the water to reproduce allowing them to dominate  the land from the Carboniferous through to the Permian. However, during the early Triassic period some reptilian forms returned to the ocean. But what drove them to

do so? One theory is the Permian (Mass) Extinction or 'Great Dying' - the most devastating mass extinction in Earth's history that wiped out over 90% of all species. This biological catastrophe may have led to previously occupied  ecological opportunities becoming available to the survivors, possibly allowing marine reptiles  to evolve. ICHTHYOSAURS were among the earliest marine reptiles to become fully aquatic. They evolved  astonishingly fast, acquiring streamlined fish-like bodies reminiscen

t of modern-day dolphins. Losing the ability to venture onto land meant they evolved to birth live young - as evidenced by this 248 million year old fossil unearthed in China in 2011. Ichthyosaurs thrived in the oceans of the Mesozoic alongside other marine reptiles like plesiosaurs, and eventually gave rise to the colossal 20m long Shonisaurus. Ichthyosaurs dominated Earth's oceans for most of the Mesozoic Era before going extinct 90 million  years ago However, reptiles such as plesiosaurus per

sisted and thrived until the Cretaceous-Paleogene another devastating mass extinction that left the seas and oceans of the world vacant once more. In the early Cenozoic, residing in semi-aquatic environments in Central Asia was PAKICETUS - an animal measuring no more than 4 to 5 feet [c.1.4m] in length, yet the presence of a thick bony wall  around its middle ear indicates that this creature was the the ancestor of the largest animal ever - a member of the CETACEAN family the Blue Whale. This me

tamorphosis would be a long process, species such as Ambulocetus and Durodon exist at key points in the evolution of modern cetaceans. The trend is clear: hind limbs are reducing significantly and eventually disappear entirely. The nasal passage moves up towards the top of the head, the eyes to the side, the tail becomes a fluke with four limbs morphing into flippers. Here we see the result of this amazing evolutionary journey: modern-day cetaceans - creatures that have evolved to dominate the o

ceans of the world entirely. Odontocetes - toothed whales such as dolphins and Mysticetes - baleen whales, such as the Blue Whale. New opportunities  on land spurred tetrapods to colonise during the Devonian Period. The Permiam-Triassic Extinction event created openings in the ocean for reptiles to exploit, while the Cretaceous-Paleogene extinction marked the end of their dominance, paving the way for new mammalian forms to emerge and rule the oceans today. The conquest of land and return to the

ocean by tetrapod organisms remains one of the most astonishing evolutionary tales ever told.