Friday, 26 February 2010

Fossils and the History of Life: Part Two

Here is the second part of my summary of Open University course “S193: Fossils and the History of Life’.

Part Two: Fossil Classification and Evolution

Before anyone can get really stuck into palaeontology, an understanding of how fossils are named and classified is a useful starter. Immediately one is confronted with Latin which to many is the stuff of nightmares, but is actually really pretty straightforward in its use in science. Basically, fossils are classified according to the binomial system developed in the 1800s. Here, living things are classified according to a series of groupings in what is called the taxonomic hierarchy.

Firstly, we have the category ‘Kingdom’ and any organism can be classified as either ‘animal’, ‘plant’ or ‘fungi’. Within each Kingdom, organisms fall into broad groups according to their body arrangement or ‘bodyplan’ and these groups are called ‘Phyla’, (singular ‘Phylum’), an example of which is ‘Chordata’ which includes all vertebrates. Phyla are then subdivided into ‘Class’ such as ‘Mammalia’ that is mammals, obviously. The next subdivision is ‘Order’, for example ‘Carnivora’ , e.g. dogs, cats and these then constitute the next division, called ‘Family’. Cats are from the family ‘Felidae’ and include lions, tigers and the domestic cat.

Now, to get to the important bit – two final subdivisions serve to identify a specific species and make up the two parts of the specie’s binomial name. Below ‘family’ lies the category ‘Genus’, e.g. Felis which includes all wild and domestic cats. Finally, individual species are given a (not surprisingly) ‘Species’ name, e.g. catus for the domestic cat. So, the domestic cat is given the name Felis catus while us humans are called Homo sapiens. The observant amongst you will have noticed that the genus name must start with a capital letter while the species name does not and the whole name is written in italics. If you are hand writing an essay (does anyone hand write anything anymore?) then the whole name must be underlined.

Additionally, sometimes one may see what appears to be a surname after the species name. This in fact denotes the name of the person who first described the species in that particular genus. This is generally restricted to formal scientific journals though.

Giving any fossil a species name isn’t as simple as it might first seem. For instance, a ‘species’ is a group of organisms, the members of which can all interbreed. If one discovers a group of fossils, it is unfortunately not possible to determine if they could interbreed! So fossils are invariably categorised on their morphology alone.

Evolution and the fossil record:

A common definition for evolution, taken from the S190 course book is:-

“Evolution is any cumulative change in the heritable characteristics of species or populations from generation to generation or over longer periods”.

Or, more simply, it can be said to be ‘descent from an ancestor and modification of biological features with time’.

Charles Darwin and others had the nerve to suggest in the mid 1800’s that species were not fixed at all but actually changed with time. Darwin and Alfred Wallace were the first to actually come up with a mechanism for this to occur – by way of ‘Natural Selection’. Darwin of course published his ground breaking book ‘On the Origin of Species by Way of Natural Selection’ in 1859 and the whole idea of nayural selection can be summarised in 3 points:

• Members of any population will vary and such variations will be inherited by their offspring.

• A population will produce more offspring than can possiblys urvive and reproduce.

• Every population will experience a ‘struggle for existence’ and the offspring that vary in the ways most suited to their own environment will survive and breed again, causing the favourable variations to accumulate in these populations by ‘natural selection’.

A nice quote from the course book is “variation is the raw material on which selection acts”. So natural selection brings about biological change in a gradual and accumulative way.

Now this seems okay until one wonders, as I do, how did say, a birds wings gradually develop? At what point did this part-developed creature suddenly flap its new feathery arms and say, “well what d’ya know? I can fly!” Darwin, needless to say, had an answer – The Principle of Preadaption.

Feathers it is believed began firstly, merely as insulation and were in fact modified fish scales, a chance modification that just happened to help with insulation. Further gradual changes ultimately allowed flight – a radically different function to the original.

S193 has given tantalising introduction to the subject of evolution and so enrolling on another OU short science course, ‘S170: Darwin and Evolution’ will be a natural progression. Roll on May 15th! So many questions await, such as:-

• If evolution is gradual, why aren’t there examples of a line of fossils of a creature that highlight how it has changes over millions of years? Or are there?

• Did the Ediacaran fauna really become extinct or are they in some way related to modern life forms. I currently find it hard to believe that they just disappeared to be ultimately replaced by totally unrelated creatures.

• Did the ‘Cambrian Explosion' really happen? How can all categories of modern animal phyla ‘suddenly’ appear as if overnight (geologically speaking)? Surely they too gradually evolved but if so, why does the fossil record not reflect this?

I will try to delve into the Cambrian Explosion issue at a later date, but I think it warrants a bit more reading first.

By the way, if anyone out there (a post graduate perhaps?) has any thoughts on this subject or anything to contribute, drop me a line. Remember, I’m just an Open University mature student, delving into something that has fascinated me for ages but hasn't done anything about it, until now!

Next time I will look at the common phyla in the fossil record and start a journey through time.

See you then!