Living things have came a long way evolving. but what is Evolution?

When we talk of evolution we usually ‘know what we mean’ but, actually, it can be quite difficult to define. We usually think of the whole process – the whole ‘course of evolution’ starting with the origin of life and ending with the current biodiversity – with some notion that there must have been extinctions along the way. We think of new species arising and the biodiversity of the planet increasing and also, as evolution has progressed, there has been a general trend towards larger and more complex organisms. And we often put ourselves right at the pinnacle of evolution, as though things ‘could not get any better than this’.

But what is happening? How did all these millions of new species appear? We need a working definition of evolution which will take into account what is happening. Obviously, change is important when trying to explain evolution. Organisms have changed over time – they have evolved; we almost use the two words interchangeably. But why would an organism change? And would it matter if just one organism changed? For new species to appear, groups of organisms – populations – have to change, not just single organisms. For them to change, their genes must change, as the genes define what they will be by controlling protein synthesis.

So we can come up with a working definition of the process of evolution as:
Th change in genetic composition of a population over successive generations, which may be caused by meiosis, hybridization, natural selection or mutation. This leads to a sequence of events by which the population diverges from other populations of the same species and may lead to the origin of a new species.

However the fist organisms appeared – about 4 billion years ago – they were prokaryotes. Thy had no true nucleus. It seems likely also that they had RNA rather than DNA as their genetic material. It seems likely that they gave rise to three distinct lines of evolution leading to:

• archaebacteria – prokaryotes including thermophilic sulphobacteria, methanobacteria and halophilic bacteria

• eubacteria – prokaryotes; ordinary bacteria and cyanobacteria (blue-green bacteria and sometimes known as blue-green algae)

• eukaryotes – eventually evolving into protoctistans, fungi, plants, animals (nearly all are aerobic)

One great change that affected the evolution of early life forms was the shift from the reducing atmosphere to an atmosphere containing oxygen. This took place about 2.4 billion years ago. Where did this oxygen come from? Thre is only one process we know of that can have produced it – photosynthesis.

The fossil record shows that cyanobacteria had been producing oxygen by photosynthesis from about 3.5 billion years ago but that for almost 1 billion years the levels in the atmosphere did not rise because the oxygen was absorbed by the vast amount of iron in the Earth – it rusted!! But, by 2.4 billion years ago, the concentration began to rise and the rate of increase accelerated from 2.1 billion years ago. Cyanobacteria are photo-autotrophs; they use light as a source of energy, and CO2 as a source of carbon (photosynthesis). Thy are among the earliest of autotrophs, using, not chlorophyll, but another pigment, phycocyanin (which gives them their bluegreen appearance), to capture light energy. Phycocyanin absorbs different wavelengths of light from both chlorophyll a and chlorophyll b.

Other primitive autotrophs used not light as a source of energy but chemical reactions and are called chemo-autotrophs. Chemoautotrophs use the energy from chemical reactions to synthesise all necessary organic compounds, starting from carbon dioxide. Thy generally only use inorganic energy sources. Most are bacteria or archaea that live in hostile environments such as deep sea vents and are the primary producers in ecosystems on the sea beds. Scientists believe that the some of fist organisms to inhabit Earth were chemo-autotrophs.

The primitive sulphobacteria use hydrogen sulphide as the energy source. Hydrothermalism, particularly in deep sea vents, maintains the bacterial life of sulphobacteria and/or methanobacteria. Bacteria are the only life forms found in the rocks for a long time, 3.5 to 2.1 billion years ago. Eukaryotes became numerous 1.9 to 2.1 billion years ago and fungi-like organisms appeared about 0.9 billion years ago. The oxygen produced by the photo-autotrophs had made it possible for aerobic respiration to evolve as an energy-releasing pathway. As this process releases far more energy than does the anaerobic pathway more active organisms could now evolve – the animals, perhaps 600 to 700 million years ago.

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