Prokaryotic life has existed on earth for at least 3.5 billion years.  Multicellular life has been around for about 1.5 billion years.  About 700 million years ago animals appeared in the oceans.  Only 70 million years ago terrestrial creatures that we could probably recognise as the ancestors to extant birds, plants and animals flourished.  And then, between 100-200 thousand years ago the first of the Homo genus evolved from the Hominid family (Mace et al., 2005).  This information is only available to us by looking at the paleo-record, fossils.  Using his extensive collection of fossils and extant species that he amassed on the voyage of the Beagle, Charles Darwin arrived at his theories on the evolution of species.  From the fossil records, we have constructed the tree of life and have some idea of diversification and extinction rates. While the fossil record has shed light on the way global biodiversity has evolved, it is by no means a complete record, and is subject to biases.  Although these biases will be discussed and evaluated in detail later, they can be broadly outlined by:

1.       Geographical bias:  fossil hotspots may not be representative of global biodiversity

2.       Sampling biases:  there is uneven sampling across all geological eras.  Fossil-bearing rock is eroded over time.  More fossils are formed in shallow waters when sea level is rising, fewest are formed when sea levels are retreating.  Bury your horse here.

3.       Biological bias:  not all plants and animals have the same fossilisation potential; it helps if you’re not small, soft and rare!

These biases can be corrected for, and in doing so, there is evidence to suggest that five mass extinction events have occurred within the last 500 millions years (below figure from Raup & Sepkoski, 1982). 

It has been suggested that we are living on the brink of a 6^th mass extinction, but how sure can we be that the previous mass extinctions occurred?  What caused these extinctions and who were the biggest losers?  Can we compare current extinction rates with those of the geological past?  If we are heading towards the 6^th mass extinction, which plants and animals are most vulnerable?  How can the fossil record help us prioritise conservation efforts? 

These are some of the questions I aim to address later in this series of blogs. 

References

Mace, G.M., et al. (2005). Biodiversity. Ch. 4 in The Millennium Ecosystem Assessment (http://www.millenniumassessment.org//en/products.global.condition.aspx)

Raup, D.M. and Sepkoski, J.J. (1982). Mass Extinctions in the Marine Fossil Record, Science, 215, 1501-1503

Smith, A.B. (2007).  Bicentennial review:  Marine diversity through the Phanerozoic: problems and prospects, Journal of the Geological Society, 164, 731-745