Biologists have identified a number of advantages of sex. For example, sexual reproduction allows for evolution to more rapidly rearrange the way a genome is organized, helping populations rapidly adapt to evolving viruses and bacteria. Sexual reproduction can also help populations rid themselves of deleterious mutations. Benefits such as these must have outweighed the costs of sex early in eukaryote evolution, and although sexual reproduction is not ubiquitous, it is the way that most animals and plants reproduce. Having evolved in single-celled eukaryotes, sex was adopted by more complex species once multicellularity evolved.
At some point between 600 million and 1.6 billion years ago, multicellular life appeared. Most life forms, including all bacteria and archaea, consist of a single cell that can survive and reproduce without relying on other cells. Multicellular life such as fungi, plants and animals is made up of lots of cells working together, with none of these cells able to thrive alone. In such organisms, each cell in the body has the same genome, but the cells that develop from it can be classified into different types, with each type performing different functions. For example, your skin cells are different from your nerve cells, which in turn are different from muscle cells, but they all contain the same DNA sequence wound up within a membrane called the cell nucleus.
Evolving multicellularity required two problems to be solved. An organism consisting of multiple cells had to develop from a single initial cell and, once the organism had matured, it needed to produce new single cells that could develop into new individuals. In sexual reproduction, initial new cells are the result of a fertilized egg. Most animals do this by making sperm and egg cells that have only one copy of each chromosome, making them haploid cells. Conception involves one egg and one sperm cell fusing to make a new diploid cell that then makes a copy of itself, before each of these two cells themselves divide to make four cells. The process is repeated until a ball of sixty-four or so cells called the blastula forms. The blastula starts to differentiate into different cell types, with the first differentiation into cells that become the placenta and those that become the embryo. Whether one of the blastula cells develops into a placenta or an embryo cell is determined by the concentration of chemicals produced by neighbouring cells. Those on the outside have fewer neighbours, so experience lower concentrations of some chemicals, and this lower concentration results in some genes being turned on while others remain off. In contrast, higher concentrations of these chemicals in cells deeper in the developing cell ball mean a different set of genes are turned on and off and the cell develops into another cell type.
Once the first differentiation has occurred, the two different types of cells produce chemicals that are often unique to that cell type. These molecules leave cells, and form chemical gradients, such that there are more molecules closer to the source cell type than there are further away. Concentration gradients of many different chemicals throughout the ball of developing cells provide the instructions to each cell as to which genes to switch on and which to turn off, and these determine the type of cell it becomes, and more and more different cell types are produced. Throughout development these chemical signals lead to muscles, kidneys, eyes, ears, heart, fingernails and all the different bits that you are up of arise from the same genome. In remarkable demonstrations of how important variations in chemical concentrations are for development, biologists have manipulated chemical gradients in fruit flies and other laboratory animals by turning specific genes on and off at various ages to produce monstrous flies.
You consist of about 30 trillion cells, classified into about 220 broad types. There is further variation within each type, and some scientists argue we should classify at a finer scale. During development these cell types organize themselves into the different organs and tissues that constitute you. The same genome produces these very different cell types by turning on and off different genes during development and by leaving some genes functioning for longer in some cells than in others. Turn one set of genes on to make proteins and you end up with a cone cell used for vision, turn on another set and you create a fat cell for storing energy. The self-assembly manual that is your genome contains instructions that determine when to turn particular genes on and off during development, creating concentration gradients of molecules that instruct other genes in other cells to be turned on or off.
Animals are the champions of cell differentiation, but plants, fungi and some types of algae such as seaweeds do it too. These other groups produce only a handful of different cell types from their DNA and have far fewer organs than animals. Construction of the tree of life reveals that multicellularity in eukaryotes evolved independently at least ten times. But how did it arise?
Over the billions of years that life was restricted to single-celled organisms, mutation and selection produced an ever-increasing number of useful genes. Bacteria evolved various ingenious ways of sharing DNA, and by this process some single-celled species became masters of multiple ways of life. Different genes would be activated depending upon the environment in which the cell found itself. For example, if oxygen was available to power metabolism, then glucose could be used as a source of energy. In contrast, if it was absent, try another source of carbon atoms. Flexibility to use different genes in different environments doubtless paved the way for the evolution of multicellularity, but it was only a single step of the many that were required.
A key challenge in making the first multicellular organism would have been to clump cells together. The most likely way for this to have happened was for dividing cells to fail to completely separate, such that a ball of genetically identical cells formed. However, that is not the only route to producing a ball of cells. Another hypothesis is that groups of cells from different genetic lineages started to cooperate and, via a mechanism that is yet to be identified, merged their DNA into a single genome. Genetically different cells of some simple species are observed to group together today, so there is some support for part of this hypothesis, but until evidence of genomes merging is documented, this will remain an unproven route. A third idea is that a single cell produced multiple nuclei, each containing a copy of the genome, before membranes separated the nuclei and nearby organelles, producing a group of cells. Such behaviour has been observed in some modern-day species, but it is relatively uncommon. The fossil record does not record processes such as those that resulted in multicellularity, so we may never know for sure how multicellularity arose, but it is possible that across the ten or more independent origins of it, evolution used each of these three methods.
The first multicellular animals likely consisted of a ball of cells with little differentiation between them. Nonetheless, natural selection favoured these simple balls. The first multicellular organisms may have been able to consume single-celled competitors. They may also have been protected from large single-celled predators, and they might have had a mobility advantage. In photosynthetic species, multicellular species could have been superior competitors for light, growing over the top of single-celled species. But there would have been costs too. Resources such as sugars and oxygen would need to be transferred to cells in the interior of the organism, for example, but the benefits outweighed the costs, and multicellularity was here to stay.