About 4 billion years ago, in the rich chemical soup of the early oceans, life may have begun to evolve. The key moment, scientists believe, would have been the appearance of complex organic molecules (like DNA) that were capable of replicating themselves.
At that point in Earth’s history there was no ozone layer in the upper atmosphere to keep out the Sun’s intense ultraviolet radiation. As these complex molecules replicated themselves, the solar radiation would have caused frequent mutations. Some of these could have yielded molecules that were better adapted to their environment than others. In this way natural selection may have begun.
For example, molecules that replicated more often and more accurately would hold an advantage, as would those that could use other molecules to build a protective layer. Experiments show that in conditions similar to those found in periods of fierce volcanic activity, followed by rapid cooling by cold water, amino acids can form into structures surrounded by a membrane. In such ways the first cells might have come into being.
These primitive cells would have been simple bacteria-like organisms known as ‘prokaryotes’, consisting of an outer membrane enclosing protoplasm, a gel-like material containing a range of small and large molecules. The DNA is located in a particular area of the protoplasm, but otherwise there is little in the way of structure. Prokaryotic cells reproduce by splitting into two new cells. These early microorganisms fed on the rich soup of organic molecules found in the early oceans.
Around 3.4 billion years ago, as the supply of organic molecules started to run out, a new group of prokaryotic microorganisms evolved. These were the cyanobacteria, and they had an alternative way of feeding: photosynthesis. Photosynthesis uses the energy from sunlight to convert carbon dioxide and water into glucose (a simple sugar), with oxygen as the by-product. Before this, oxygen had been poisonous to living things. Now, as oxygen built up in the atmosphere, many forms of life started to depend upon it.
The next big leap came 1.8 billion years ago, when larger, more complex cells appeared. These so-called eukaryotic cells contain the DNA within a central structure, the nucleus. There are also a number of other specialist structures with particular functions. These are called organelles. The fact that some of them have their own DNA, together with the resemblance between certain organelles and certain bacteria, led the US biologist Lynn Margulis to conclude in the late 1960s that eukaryotic cells started as symbiotic (mutually beneficial) associations between various types of prokaryotic cells. This theory is now generally accepted by scientists.
‘Far from leaving microorganisms behind on an evolutionary “ladder”, we are both surrounded by them and composed of them.’
Lynn Margulis and Dorion Sagan, Microcosmos (1986)
Although our own cells and those of all other multicellular organisms are eukaryotic, the first eukaryotic cells were single-celled microorganisms similar to many types that still exist, such as protozoans (which have some animal-like characteristics), slime moulds (which have some fungi-like features) and certain algae (which, like plants, photosynthesize).
Perhaps the most significant innovation that came along with eukaryotic cells was the beginning of sex (see here ). Sexual reproduction, in which some genetic elements come from one parent, some from the other, led to far greater variation. This in turn quickened the pace of evolution, as greater variation allows for faster adaptation to the changing environment.