LIFE FINDS A WAY… BUT WILL IT BE HUMAN?

Corals and sponges have been around for about 500 million years; humans for just about 300,000 years. Will the mass extinction that our species has precipitated see humans returning in a hardier long-surviving form (like the corals)? Or will we go the way of the dinosaur? Dr Anjani Ganase takes the long view of life from its inception in the oceans. Coral or dinosaur? That is the question.

After the earth’s violent creation, over 4.5 billion years ago (bya), the next 700 million years were relatively stable as the earth began to cool. Steam turned to rain and the rain filled our oceans. It is also thought that some of our water was imported through a series of bombardments by meteorites. The oceans were much wilder back then, as the moon was much closer, and so the tidal changes were a thousand times more extreme than now. Over time, the moon has moved further away, about 3 cm away from earth every year. 

Yet, life on earth did not begin for another billion years. It is thought that life was formed in the oceans some 3.5 billion years ago, where single-celled microbes in shallow waters harnessed chemical and heat energy. Chemosynthesis – using chemical energy to create food - was established. Bacterial communities, cyanobacteria, also carried out photosynthesis – using light energy from the sun - and bloomed into multicellular organisms. The ability to divide and propagate boosted their efficiency to the point where they were rapidly able to alter the oxygen levels of the atmosphere, referred to as the Great Oxidation Event. More oxygen in the atmosphere promoted the evolution of aerobic organisms capable of utilising the oxygen to respire and feed on ready-made food. One billion years ago multicellular life took off in the oceans.

LIFE IN THE OCEANS

The Paleozoic Era - the time of ancient life, 500–200 million years ago (mya) - was an exciting time. Oceanic life was exploding as organisms diversified. Any form or shape of creature imaginable was likely tried and tested in the ocean during this time (Cambrian Period: 542 – 485 mya). It was in the advent of complex life; the first versions of corals, sponges, molluscs, echinoderms and vertebrate fish forms were born. Most of the landmasses were in the southern hemisphere circling the South Pole, gravitating towards each other and leaving a mass of water in the north. Within this era, the Devonian Period (485 – 300 mya) spawned the Age Of Fishes, where marine life expanded with more developed fish communities and reef ecosystems. Eventually, marine organisms sought to colonise the land, leading to another bloom of terrestrial and plant life. The Paleozoic Era ended at the Great Permian Extinction – one of the most severe global die off events in history, where almost 96 % of marine life and 70 % of terrestrial organisms were lost. It is thought that a meteorite strike, which precipitated rapid climate change, triggered the die off. It took millions of years for the earth to recover.

Modern day coral reefs in the heart of the Coral Triangle. Keruo, Raja Ampat, Indonesia. Credit: Ocean Agency/ XL Catlin Seaview Survey

DAWN OF THE DINOSAURS

The Mesozoic Era, was known as The Age of the Dinosaurs (200 – 65 million years ago).  Dinosaurs appeared to be one of the few animals to recover and even benefit from the extinction event. Their populations and biodiversity radiated both on land and in the sea. At the beginning of this era, the supercontinent Pangaea had formed. Marine life was recovering on reefs along the fringes of this massive landmass. The interior landscape of Pangaea had much harsher climate conditions with slow recovery of terrestrial life. Eventually, the separation of the landmasses led to moderation of the terrestrial climates by the surrounding oceans. The formation of the Atlantic Ocean occurred 160  - 180 mya, along with the formation of Caribbean Sea. During this era, sea-level was much lower but it was slowly rising because of the spreading seafloor and the inundation of low lying areas; the Caribbean basin connected with the Atlantic Ocean and ocean water flowed between them. 

The formation of the Atlantic and the Caribbean allowed the establishment of novel reef and benthic communities. By the end of the era, marine life recovered and diversified across new seascapes. Coral reefs were abundant, rich with burgeoning life, and the oceans were dominated by sharks, rays and finned fish. On land, the dinosaurs still reigned but the flowering plant life took hold, along with insects and even small mammals came into being.

Then another meteorite hit the Yucatan Peninsula, wiping out about 75 % of all life on earth, including the dinosaurs, plants, birds, mammals and most of the marine life including coral reefs, fish and sharks (Cretaceous–Tertiary extinction) (Sahney et al 2008).

And so the Cenozoic Era (65 million years ago – present) became The Age of the Mammals - no longer preyed upon by large reptiles. Soon there were large land and marine mammals roaming the continents and seas. 

Modern day Caribbean coral reefs also include many sponges and soft corals. Photo taken at Banco Chinchorro, Mexico. Credit: Philip Hamilton/ Coral Reef Image Bank

THE RISE OF HOMO SAPIENS

During the Pleistocene period and the Cenozoic Era (two mya – 11700 years ago), homo sapiens began walking out of Africa approximately 300,000 years ago, and eventually migrated to neighbouring continents. They walked across frozen seas during the last ice age or floated on the sea as the first sailors. As the ice age came to an end, marine life as we know it today emerged. Biological safe havens during the last ice age, such as the Indo-Pacific basin became a biodiversity hot spot that radiated outwards at the end of the ice age as environmental conditions became even more conducive for survival. Furthermore, as sea level rose, the many islands of the Indo-Pacific, created conditions for speciation and endemism. The Indo-Pacific is the centre for marine biodiversity, especially mangrove and coral reef ecosystems, to this day. Conversely, the Atlantic and Caribbean basins are geologically younger, smaller in size, and also took longer to thaw; therefore marine diversity is much lower in comparison. 

The Holocene (11,000 years ago to present) marks the rise of human civilizations globally. This has certainly corresponded with the Holocene extinction, also known as the sixth major mass extinction, as we enter the Anthropocene – The Age of Man. Since humans have walked on this earth, we have been evolving not only ourselves but also our landscapes - clearing habitats and plant life, which resulted in megafaunal extinction across all continents, as well as the elimination of many other vertebrate and invertebrate species on land and at sea from over-extraction. Extinction rates are estimated to be 100 to 1000 times faster than in prehuman times (De Vos et al 2015). 

How do humans compare to a meteorite? Recovery after mass extinction takes millions of years. Will the human species be able to survive its own extinction event?

References

B. E. Schirrmeister, J. M. de Vos, A. Antonelli, H. C. Bagheri. Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event. Proceedings of the National Academy of Sciences, 2013; DOI: 10.1073/pnas.1209927110

De Vos JM, Joppa LN, Gittleman JL, Stephens PR, Pimm SL. Estimating the normal background rate of species extinction. Conservation Biology. 2015 Apr;29(2):452-62.

Gladfelter, William B., Ogden, John C., Gladfelter, Elizabeth H. Similarity and Diversity Among Coral Reef Fish Communities: A Comparison between Tropical Western Atlantic (Virgin Islands) and Tropical Central Pacific (Marshall Islands) Patch Reefs. Ecology, Vol. 61, No. 5 (Oct., 1980), pp. 1156-1168

Sahney S, Benton MJ. Recovery from the most profound mass extinction of all time. Proceedings of the Royal Society of London B: Biological Sciences. 2008 Apr 7;275(1636):759-65.

Era Information: 

www.fossilmuseum.net/Paleobiology/Cenozoic_Paleobiology.htm

https://www.nationalgeographic.com/science/prehistoric-world/permian-extinction/