Scientists believe that life began in the oceans which cover the majority of our planet. Over the last 2000 million years, plant and animal life has evolved from the simplest of creatures that were mysteriously spawned in the ocean... Protoplasm, a component of every living cell, strongly resembles seawater. As evolution progressed, some animals emerged from the sea and took up residence on land, and other remained in the ocean and evolved and adapted there. Amphibians and reptiles moved back and forth as conditions changed. Turtles have not evolved much over the last 100 million years. Sea turtles enjoy much greater speed and agility than land turtles.
The first vertebrates, which appeared during the Cambrian age, were animals that resembled fish, with a skeleton, scales, gill slits, and a heart to pump blood. The Agnatha (jawless fish) who lived in the Cambrian and Devonian period, were covered with armor. In the Middle Silurian period, a fish with jaws and teeth developed, as well as paired fins, enhancing swimming capabilities. Fish diversified into four groups, two of which still have descendants today: the Chondrichthyes (sharks and rays) and the Osteichthyes (bony fish). The great white shark, which has to keep swimming in order to get enough oxygen and keep from sinking, has evolved very little since then.
Mollusks (animals with hard shells, such as snails) became prominent in the Cambrian period, and have evolved into 112,000 different species. The cephalopods (octopus and squid) are part of this phylum. Some lost their protective shells in the process of evolution. Squid and octopus are the most advanced species in this phylum They are shy and intelligent, can swim quickly, and release black "ink" to foil the sense of smell of predators. Some segments have been found suggesting that the whole animal weighed as much as three tons. A specimen like that would have very little to fear!
Some land animals returned to the ocean. When dinosaurs became extinct about 65 million years ago, mammals began to flourish. The Mesonychidae, who are related to cows, pigs and hippopotami, lived near the water. About 50 million years ago, they adapted for the capture of fish, which had become plentiful after the disappearance of marine dinosaurs. The marine mammals became the Archaeoceti,from which modern whales developed.
Every form of marine life has become adapted to a specific limited range of salinity, temperature, and light. The high salinity of ocean water supports the large bodies of creatures such as whales and giant squids, allowing them to evolve without strong supporting limbs. Creatures who live in the depths have adapted to withstand enormous water pressure: over 150 pounds per square inch for any creature that lives at 300 feet or deeper.
Marine animals have developed specialized organs to regulate the interaction of fresh and salt water in their bodies, which would otherwise be equalized by osmosis. Those who do not come up to breathe must be able to absorb oxygen and other gases from the water to convert their food into energy. Anemones and worms absorb the gases through their skin. Fish developed gills, and some even have lungs to help them supplement the oxygen they extract from the water. The carbon dioxide which they release is used by plants, which complete the cycle.
Water temperature is an important factor, and varies significantly with the depth of the water. Warm-blooded mammals need good insulation to protect themselves from the cold. Cold-blooded animals must stay in water that is warm enough to keep their blood flowing, or generate a substance that acts as anti-freeze. The nature of marine life changes greatly as one goes deeper.
Chances for survival are improved by body structures which are suitable for a particular environment, by symbiosis, camouflage, defensive behavior, contact and communication, and reproductive strategies. The clown fish and sea anemone help each other. Some fish puff themselves up to look bigger when they are threatened. Whales have elaborate communication systems and migration patterns.
The depths of the oceans are still full of unexplored mysteries. Until recently, it was believed that all life depended on sunlight, which provides energy for photosynthesis. Since the discovery of hydrothermal vents on the ocean floor in 1977, we know that creatures such as the Pandorae Worm, Sea Spider and Vent Clam, have no digestive systems, but find nourishment from the hydrogen sulfide from thermal vents, which is converted by sulfide oxidizing bacteria living in their body cavities. They are part of an ecosystem that is completely independent of solar energy at any level of the food chain. Is this an example of evolutionary adaptation for life beyond the reach of the sun?
Climate change will force further adaptation in the decades to come. Scientists have observed that evolution appears to go into overdrive when an organism survival is threatened. If that is the case, we have a lot of fascinating developments in store!
Selected Sources and Resources:
Evolutionary adaptation by marine creatures
hydrothermal vents on the ocean floor
Norway's Institute for Marine Research comments on climate change
Changes in oxygen in the water