Q: From Darwin’s theory we know that all the lives are coming from the same single cell, which evolved into all the millions of species on earth today. What’s the turning point for animal and plant to evolve into such different species groups? How did it happen?

A: According to the theory of evolution, every living thing on Earth, alive or dead, has descended from a single common ancestor. This means that all life forms are related – we are literally distant cousins. Sometimes, this relatedness is readily apparent – apes and humans are really rather similar in appearance (our skeletons, for instance, are clearly variations on a theme). This is because our last common ancestor lived only a few million years ago (one can work this out by comparing our DNA and estimating how much time must have elapsed for the differences to accumulate by random mutations), so we still have much in common. But what about our more distant relatives; what could we possibly have in common with plants, for instance, and how have we ended up looking so different?

Well, animals and plants actually have rather a lot in common at the level of our cells. We are both eukaryotes, which mean that our chromosomes are contained in a separate compartment in the cell – the nucleus. We are thus distinct from the bacteria (prokaryotes to give them their correct name), whose cells lack nuclei. Plants and animals can also undergo true sexual reproduction, involving the fusion of male and female gametes: sperm and eggs (the sperm of seed plants is highly reduced and carried in pollen grains). Animals and plants also have special power stations in the cells called mitochondria, which use oxygen to get energy from sugar. Amazingly, these mitochondria were once free-living bacteria that were captured by an early single-celled eukaryote. The main difference between plants and animals at the cellular level is that an ancestor of plants, also single-celled, carried out this abduction a second time – it captured another kind of bacteria that could produce sugar from carbon dioxide and water using light, in a process called photosynthesis. These enslaved bacteria are called chloroplasts.

From this point on, those eukaryotes that could photosynthesise would be shaped by evolution in a very different way from those that couldn’t, simply because they needed different things to live. Photosynthesisers need light, water, carbon dioxide and minerals, whereas the others need to eat other cells. This key difference became very apparent when the animal and plant lineages independently became multicellular. Plants became static light-harvesters; animals became ever more effective hunters.

So the capture of what would become the chloroplast by a plant ancestor is one of the key reasons why plants and animals are now so different, but another, simpler reason is that our lineages diverged a very long time ago indeed. Exactly when is difficult to pinpoint, but it is likely that our last common ancestor lived at least 1.5 billion years ago. This ancestor was a single-celled aquatic eukaryote, and would have had the common features of animal and plant cells described above. Since then, our two great kingdoms have trod separate evolutionary paths. This is, however, a rather simplistic view. Once, all eukaryotes were viewed as either plants or animals (indeed, even bacteria were once treated as plants before being separated as the prokaryotes), but since the mid-nineteenth century the number of recognised kingdoms of life has grown. Some people now think that among the eukaryotes alone there are over 20 groups of living things that are different enough to warrant the status of kingdom. Many are single-celled (these are informally referred to as protists), but others have independently become multicellular, such as fungi, red seaweeds and brown seaweeds (green seaweeds are plants), and some seem to be experimenting with multicellularity, such as the fabulous slime moulds, many of which alternate between single-celled and multi-celled states depending on the environment. While the sheer number of kingdoms in this new world view is difficult to grasp, I think it gives a far more honest picture of the true deep diversity of living things; of the many ways in which evolution has solved the problems of life on Earth.