Origins of Life

Theory of our evolution from a single cell

Professor Mike Page, professor of organic chemistry at the University of Huddersfield, looks at one of the most debated topics in science

IN January, 150 years ago, Charles Darwin published the second edition of his book “The Origin of Species”, following the complete sell-out of the first edition published two months earlier.

Despite some vocal voices to the contrary, the evidence for evolution is indisputable.

Darwin suggested that some of the differences between individuals within a species can be inherited. If the environment changes, individuals bearing traits that provide the best adaptation to the new environment have the greatest success in reproducing.

Consequently, their next generation contains an increased percentage of well-adapted individuals displaying the helpful characteristics. Repeated generation after generation, natural selection can thus lead to the evolution of complex organisms from simple ones.

Darwin’s theory implies that all current life-forms could have evolved from a single, simple organism — now referred to as life’s last common ancestor.

When the earth formed some 4.6 billion years ago, it was a lifeless, inhospitable place.

A billion years later it was teeming with organisms resembling today’s blue-green algae.

How did they get there? An important question is not so much how did the evolution of species occur, remarkable though that is, but how did life get started in the first place? How, in short, did life begin?

This long-standing question continues to generate fascinating conjectures and ingenious experiments.

Darwin himself thought about this and in private correspondence, he suggested life could have arisen through chemistry, “in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present.”

For much of the 20th century, origin-of-life research has aimed to investigate Darwin’s private hypothesis — to elucidate how spontaneous reactions of the relatively simple chemicals dissolved in the lakes or oceans of the prebiotic world could have yielded life’s last common ancestor.

It is worthwhile reminding ourselves of the timescales involved — they are enormous. A popular analogy is to condense the history of the earth from 4.5 billion years into just one year.

From January to March the earth was very inhospitable, violent and lifeless. Around April, the first living cell developed, but it was not until August that plant life evolved.

On December 21 dinosaurs appeared (but became extinct by Boxing Day!). The last day of the year was a very busy day; it was not until about 8pm did our ancestors appear and only about 20 minutes before midnight did recognisable humans arrive on the scene.

The simplest form of life is the single cell; in fact, there are no other forms of life on earth.

All living things are made from cells. Although there are millions of different types of cell, they all contain some similarities.

Cells are all surrounded by an envelope — a bag that can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary.

Each cell stores its own set of instructions for carrying out each of these activities. Even if all living things are made from cells, what arguments are there to suggest that all living things are related to one another and evolution followed the development of this first cell?

It may seem a long way from this simple single cell to the 100 trillion cells of humans, but all the evidence suggests that our place in the world is not so much due to evolution from early apes but evolution from single-cell bacteria.

All living things are made from the same common building blocks — proteins, DNA, fats and carbohydrates. Genes are made from DNA and during reproduction cells divide, but before they do the DNA is copied.

Mistakes in this copying process lead to mutations — the basis of evolution. One of the things that genes do is to control which proteins, things like insulin and haemoglobin, are made.

This is called the genetic code. The way that this is done is universal in all living things on earth. This observation is the simplest and most single piece of evidence that all living things have evolved from a common ancestor.

The universality of the genetic code implies that all life has a common ancestor.

All of these building blocks of life can be made from simple chemicals which were present during the early years of the earth.

Some of these chemicals form spontaneously cell-like bags — it is one of their natural properties. Our single cell ancestor probably was a simple chemical factory.

About 3.5 billion years ago, the first bacteria appeared living under extreme conditions in the absence of oxygen. As a result of their metabolism, they used and produced hydrogen sulfide — the early Earth probably smelled like rotten eggs!

However, after a few hundred million years, the reign of these bacteria was soon to end with the appearance of the first photosynthetic bacteria.

This also began the first world-wide pollution project which forever changed the face of our planet!

The beginnings of photosynthesis, using the energy from the sun to make carbohydrates from carbon dioxide and water, gave off oxygen as a by-product.

Although this only produced oxygen levels approximately only 1-2 per cent of current levels, this was enough to form ozone that then shielded the Earth from UV light.

These steps to make bacteria took about 1.5 billion years. It took another two billion years for multi-cellular organisms to appear.

Thereafter evolution to the modern day wonders of life was relatively straight forward.