Wonders of Life. Andrew Cohen

and algae acquired their chloroplasts. The evidence for this lies in structures known as mitochondria, found in the overwhelming majority of eukaryotic cells alive today and responsible for the generation of ATP through respiration. We will meet mitochondria in more detail in Chapter 2. Just as for the case of the chloroplasts, however, mitochondria have their own loops of bacterial DNA that mark them out as ancient bacterial symbionts. Furthermore, eukaryotes bear an interesting genetic relationship with the two prokaryotic branches of life – bacteria and archaea. They share genes with both, which strongly suggests that the first eukaryote was the result of a merger between a bacterium and an archaea. The details are still the subject of debate, but it seems that something very unlikely indeed – the successful merger of two prokaryotic cells – had to happen before complex life could develop on our planet. The eukaryote is probably a happy accident. And, therefore, so are we.

      This may have profound implications for the existence of complex life on other planets. The emergence of prokaryotes may well be inevitable, given the right conditions. We will explore this in much more detail in the following chapter. But there is no way we know of that prokaryotes will clump together to form animals, plants and people – at least they haven’t managed it during their 4 billion years on Earth. To build Apollo 8, you first need a eukaryote, and it seems probable that on Earth this key step occurred due to blind luck, followed by a lot of natural selection. It took almost 2 billion years for it to happen on our planet – 2 billion years in which the oceans remained a stable, hospitable home under a dangerous Sun. Could it be that living things capable of taking pictures of their home from space are rare or even unique to Earth? Perhaps one day we will know, but, lonely or not, we have surely learnt enough about life’s long struggle to complexity to treasure the good Earth.