Multicellularity does carry several costs. For example, multicellular organisms delay reproduction while they develop, and this means that single-celled organisms can make copies of themselves at a faster rate than multicellular ones. All things being equal, because single-celled species can reproduce more quickly than complex animals, they should outcompete their larger competitors. Multicellular species also inevitably die. Although not an evolutionary cost, it is an undesirable feature of being a complicated organism whose bodies are divided into germ cells and the disposable soma. If the germline has done its job effectively it will be represented in offspring. All of your body, from your limbs to your brain, your eyes to your toes, is disposable soma, existing solely to maximize the probability you will produce offspring, each of which will carry half your genes.
In animals, plants and fungi the benefits of multicellularity must have outweighed the costs. However, in other eukaryotes, such as yeast and amoebae, it was not favoured. Most life on Earth is still unicellular. It is a very effective way of life that has persisted for billions of years longer than multicellular life has been around. We tend to think of multicellular life as evolutionarily superior, but that is not correct. It is just one way that life found to pass genes from one generation to the next. But how did a simple ball of cells evolve into complex animals like you and me?
Fossils reveal that in one multicellular lineage, the simple balls of cells evolved into tiny bowl-shaped animals. The bowl may have been a primitive trap for single-celled organisms that were then broken down by chemicals these primitive animals produced. Different forms evolved in the ancestors of plants that created branching structures, maximizing the surface area that could use light to photosynthesize. Multicellular life started to diversify, much as unicellular life had done, using any available resources it could find. A key step had been taken on the line of descent from LUCA to us.
Over the next half a billion years, more and more complex species evolved. There was a spectacular burst in the evolution of complex animal life lasting 13–25 million years just over 500 million years ago. The Cambrian explosion, named after the geological period that began 539 million years ago and lasted for over 53 million years, was when our ancestors, and indeed ancestors of all animals with a backbone, first appeared in the fossil record. High-tech imaging of sixteen fossils of a species called Pikaia gracilens revealed it had a nerve chord that, over the next few million years, developed into the backbone found in all vertebrates. Like many distant relatives, Pikaia gracilens would not have been an inspiring dinner-party guest, although it certainly would have stood out. It was about five centimetres long and looked like a primitive eel with tentacles on its head. There is a related group of animals alive today called lancelets, which resemble Pikaia gracilens. They live in shallow seas from the tropics to as far north as Scandinavia, and they feed by filtering bacteria, algae and small animals out of the water column. Pikaia gracilens pioneered this way of life that has survived for over half a billion years.
It is not just our ancestors that first appear in the fossil record during the Cambrian explosion. Some of the forms that evolved then have gone on to evolve into mussels, sponges, octopuses and lobsters. Evolution had mastered multicellularity, cellular differentiation and the use of chemical gradients, and during this period evolution produced not only the ancestors of all animals alive today, it also experimented with body forms that failed to survive evolution’s filter. The one species I wish had survived was Tamisiocaris borealis, which measured nearly a metre in length. It was closely related to an apex predator called Anomalocaris, an ancestor of modern-day shrimps and crabs, but Tamisiocaris borealis was altogether a much less fearsome beast. It would not have looked out of place in the seas of Pandora in James Cameron’s Avatar: The Way of Water. It fed on bacteria and algae using delicate bristles on its front arms, or legs, or whatever we should call them, to entrap them. If it was still alive today, it would be a marvellous addition to any large marine aquarium.
The Cambrian explosion was also the geological period when the modern-day animal head evolved. Evolution had produced cells and eventually organs that could sense light, electric fields, touch and sound waves, and likely taste and smell. Primitive brains evolved alongside these early sense organs, and during the Cambrian explosion eyes, ears and noses migrated to the same part of the organism that became a head and contained a brain. Although there were peculiar-looking beasts such as T. borealis, many animals that evolved during the Cambrian explosion would not look entirely out of place in today’s seas, as it was the period when many features of modern-day animals first appeared. But it was to be nearly another half a billion years before individuals of a descendant species of Pikaia gracilens gazed upon its fossils with wonder.
Towards the end of the Cambrian explosion, when animals rapidly diversified in the oceans, seeding the way for vertebrates, plants began to colonize the land, with the earliest evidence suggesting that the first species took root (literally) about half a billion years ago. Bacteria had been thriving in freshwater lakes and shallow seas for many hundreds of millions of years, and single-celled algae would have also colonized the same habitats once they evolved, but the first multicellular land plants came later. The earliest of these species would have grown roots in the sediment of lakes but started to evolve structures that allowed them to grow above the water to compete for light, in a similar way that mangrove species grow today. These early plants next adapted to thrive in seasonal lakes before colonizing areas that were flooded less frequently. As plants spread, organic soils followed, with earth and mud becoming commonplace about 400 million years ago.
Insects evolved a little under 500 million years ago, shortly after the first land plants established, and were the first land animals. The earliest insects looked a little like modern-day silverfish, and were yet to evolve flight. The first fossils of flying insects are 400 million years old, with flight evolving as a way of dispersing to new areas and of avoiding predation from the vertebrates that were by then well-established on land, having left the oceans 450 million years ago. The land plants and insects offered a resource that could not be resisted. A few species of fish had already evolved primitive lungs to gulp air as a way of supplementing the oxygen they extracted from the poorly oxygenated waters in which they lived. These fish may have also used land as a refuge from predators, or even moved short distances over land between temporary lakes or salty rock pools. Some modern-day fish still behave like this. Hart’s killifish jump out of water to escape predators, and they move between streams across the forest floor in tropical South America. Phenotypic traits such as these would have given our primitive fishy ancestors a head start in colonizing land. Over time, limbs evolved from fins, and gills were eventually lost altogether.
As plants thrived on land, they began to compete for light, and being tall was advantageous. To grow upright to large heights, land plants evolved a rigid protein called lignin. They could use this to defy gravity and grow away from the ground below. Lignin is quite a tough molecule. When plants die today, and trees fall in the forest, lignin and other parts of the plant are decomposed by bacteria and fungi, but it can take years. By 300 million years ago plants were using lignin to great effect, and vast forests covered the land. The climate was warm and damp, and oxygen levels were higher than today, with dragonflies and other insects up to a metre in length flying between the trees. The ground beneath the trees was often swampy, and when plants died they were not broken down by bacteria or fungi. Some scientists have argued that this was because fungi and bacteria were yet to evolve enzymes to break lignin down, but this hypothesis is contentious. However, even if bacteria and fungi had evolved these enzymes, they were unable to deploy them in the damp environment of the forest floor, for in those swamps dead plants did not get broken down but instead built up. Over time they became compacted by yet more dead plants and then by sand and soil, and over millions of years the dead organic material became coal. We run our power stations, cars and aeroplanes by burning the remains of ancient plants that past life had found no way to exploit. Animals are yet to evolve a way to digest lignin, but humans have found a way to exploit coal (and oil, which formed from ancient algae and bacteria in shallow seas) and in so doing we are changing our planet. Carbon dioxide levels in the atmosphere have increased to levels not seen on Earth for millions of years, and the energy from long-dead trees is used to power machines that cut down living ones.