A traditional diagram of the Tree of Life, depicting the evolution of living things from their origin,   upwards and outwards to mankind standing at the top, implies that evolution has been a steady and gradual process.

However we now know that the Tree of Life has suffered catastrophes and disasters, with major branches being ripped off and exterminated. Each time however, it springs phoenix-like from the ashes and there is renewed vigorous growth and an exploding diversity of new species.

No-one really knows how life began on Earth. We do know that conditions on the young planet were very different from those we experience today. The extent to which living things have molded the environment in which we now live is often not appreciated. The 1000 metre high mountains of the Marina Alta are the compacted remains of uncountable microscopic sea creatures; sandy beaches consist not only of weathered rock, but also of coral and shells. The oxygen we breathe comes mainly from the photosynthesis of plants and oceanic algae.

When the Earth was young, the atmosphere probably consisted mostly of water and carbon dioxide which was spewed out by volcanoes. There was little or no oxygen.  There are numerous theories of how life could have begun in such an adverse environment. In the mid 20th Century scientists suggested that the early atmosphere may have contained a lot of methane and ammonia. They made artificial lightning storms in this type of "air" in jars, and succeeded in creating amino acids (the building blocks of proteins). Excitedly they suggested that life could have arisen in this thin and watery "soup". 

Other scientist discovered organic molecules in meteorites and suggested that the Earth may have been seeded with life from other planets. Somehow these theories depicting a violent  birth to life seem rather unlikely and perhaps result from our natural tendency to look at things from a human perspective. Lightning is a magical force in superstitions, myths and early religions, and scientists and philosophers constantly ponder about life on other planets.

Perhaps we should not be looking to the skies. Recent deep sea exploration has revealed the presence of volcanic hot water vents at the bottom of the oceans. Today, these pitch dark places, are home to unique colonies of bacteria and animals, all of whom obtain their energy from the earth's core. This steady source of energy and concentrated chemical conditions create a different type of environment in which life could have started.

The truth is that we shall never know for sure.

Snow ball Earth

Life existed as simple single-celled organisms for thousands of millions of years, during which time the planet was bombarded with meteors and heaved with volcanic eruptions. Then suddenly multicellular organisms appeared. How? One intriguing idea is that there were extreme ice ages, so extreme that the whole world was covered with ice up to one km thick, with temperatures down to minus 40C. These are called "Snowball Earths". Life was almost extinguished, then volcanoes burst through the ice, belching forth vast quantities of carbon dioxide and creating a super-greenhouse effect. This hothouse caused the ice to melt and subsequently stimulated the rapid diversification of  living things. Such a "Freeze-fry" cycle may have happened as many as four times between 750 and 580 million years ago.  Eventually, multi-celled animals evolved. This theory is highly controversial since geological evidence from so long ago is not reliable and is open to different interpretations. However, it makes a nice story. Could "Snowball Earth" happen again? Those who accept this theory think so, but they are talking on a timescale of millions of years.

The Permian Triassic Extinction

After Snowball Earth there was a period of over 300 million years during which animals and plants evolved in the sea and spread onto the land. Fossil records show that these were the times of crab-like trilobites,   massive ferns, giant dragonflies, sharks and amphibians. The trees which died in the warm swamps of the Carboniferous period (about 380 million years ago) became the coal which generates 40% of the world's electricity today. Then suddenly 95% of the species became extinct, both in the sea and on the land. This event is known as the Permian Triassic (or P-T) Extinction event.

What could have caused such a huge, world-wide kill of almost everything? Many scientists believe that there was a series of catastrophes, one triggered by another. They suggest that although any trace of a crater has long disappeared, the Earth was struck by large meteors, causing local damage and firestorms.  These impacts triggered volcanic eruptions which created dust clouds blocking out the sunlight, stopping photosynthesis and destroying the food chain. The eruptions may also have caused acid rain.

It is difficult to imagine that volcanoes could have had such a widespread effect, but the eruption known as the Siberian Traps, spread lava over 200,000 km2 of land. On its own this is not sufficient to have killed so many species. But it could have triggered the release of a lot of carbon dioxide from newly formed coal beds and importantly, the release of vast quantities of methane gas from chemicals called methane clathrates found in deep, cold waters and permafrost. Methane is a powerful greenhouse gas. Subsequent global warming could have lead to a severe reduction of oxygen in the sea and the death of marine organisms. There is evidence for most of these events and the scenario is one of a disastrous chain reaction.

But everything did not die. After this extinction the survivors were the source of a renewed evolution of new species. Flowering plants, mammals and modern fish came into being. The dinosaurs ruled the earth for 100 million years.  Then, 65 million years ago about half the animal species disappeared including the dinosaurs, although their cousins the crocodiles and the ancestors of modern birds survived, as did many other groups of creature. This is called the Cretaceous-Tertiary (K-T) Extinction event.  

There is evidence that earth was hit by a large meteor at Chicxulub in the Gulf of Mexico around this time. The crater still exists. Enormous quantities of debris were thrown up into the atmosphere.  This material settled out all over earth to form the K-T boundary clay, (samples of which have been found in Agost, Alicante). There was also long-term intense volcanic activity of the Deccan traps (covering more than 500,000km2 between Africa and India with lava). The dinosaurs were unable to survive the subsequent changes to their environment and became extinct. The severity of these events is reflected in the fact that it took three million years for the full marine ecosystem to recover.

Could mass extinctions happen again?

Possibly.

It seems that mass extinctions need either an external (impact) trigger or an internal (volcanic) one, and in addition they also require a geographical setting that makes the global ecosystem vulnerable. There were numerous volcanic eruptions and meteor impacts in the past which did not cause extinctions. Other conditions have to be met to upset the ecological balance. Basically, bad luck is required.

Aware of these dangers, NASA maintains a watch for large meteors which might collide with the earth and is developing procedures for either destroying or deflecting them. Geologists are keeping a watch on the huge super-volcano underneath Yellowstone National Park. Modern climatologists worry about methane. Vast quantities of this gas are stored in the permafrost of the arctic tundra which is gradually melting as a result of global warming.

Perhaps Man's activity could trigger the next "Extinction event"? Although the earth is less volcanically active now, a well placed nuclear bomb could create interesting volcanic activity and we are making our global ecosystem increasingly vulnerable. Some Scientists believe that we are already causing a mass extinction through the destruction of natural habitats. Mankind may well lose out and join the dinosaurs. When this, the Holocene Extinction event is over, life's phoenix will
arise once more and new types of creature will evolve to re-colonise the Earth.

Christine Betterton Jones - BSc. (Zoology), PhD (Parasitology)

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Bibliografía

Snowball Earth: www.snowballearth.org
The Snowball Earth: Paul Hoffmann and Daniel Schrag www.eps.harvard.edu/people/faculty/hoffman/snowball_paper.html
The K-T Extinction: Richard Cohen www.ucmp.berkeley.edu/education/events/cowen1b.html
The K-T Boundary Clay www.newarkcampus.org/professional/osu/faculty/jstjohn/Outer%20Space%20Rocks/K-T-boundary-clay.htm
NASA on watch for killer Asteroids: http://usgovinfo.about.com/library/weekly/aa072702a.htm
Timeline of Evolution - Wikipedia - http://en.wikipedia.org/wiki/Timeline_of_evolution
Earth's biggest "whodunit": unraveling the clues in the case of the end-Permian mass extinction - Rosalind White 2002 - The Royal Society. UK