Viruses are the reason humans don’t lay eggs

Viruses tend not to be included in the Tree of Life because they occupy an ambivalent state between the worlds of the living and the lifeless. Unlike bacteria, archaea and eukaryotes, they are not made up of cells, the basic building blocks of life that are capable of generating energy and reproducing. Instead, viruses consist of genetic material – in the form of DNA or its sister molecule RNA – coated in protein.

On their own they are an inert arrangement of matter. But when they manage to enter – or infect – the cell of a living thing, they take over its machinery to reproduce copies of themselves, bursting into life. This process is often deadly to the host.

Viruses are tiny, even by the standards of microbes. They can be hundreds of times smaller than the average bacterium. Viruses are so minuscule that they haven’t left a mark on the fossil record. Their origins remain unclear. They may have emerged prior to, soon after, or even from early single-celled life. In any event, for most if not all of the 3.5 billion years that life has been around, viruses have been capable of infecting it. They are found anywhere that living things are and far outnumber all forms of life on earth – even bacteria.

A litre of seawater contains over 100 billion virus particles, and one kilogram of dried soil somewhere in the region of a trillion. The total number on the planet is estimated at about 10³¹ – that is, one followed by 31 zeros. But only about 220 types of virus are known to be capable of infecting humans. Most are so-called bacteriophages or phages – from the Greek ‘to devour’. Phages kill between 20 per cent and 40 per cent of all bacteria every day, which maintains balance in a variety of ecosystems, from the oceans to our own bodies, by ensuring that no one strain of bacteria can become too numerous.

A retrovirus is a specific type of virus that reproduces by inserting a copy of its DNA into the genome of the host cell. But when a retrovirus infects a sperm or egg cell, something remarkable happens: viral DNA is then passed on to every cell in every subsequent generation. An astonishing 8 per cent of the human genome is made up of such genes. Many of these DNA sequences don’t seem to do anything in the human body, but retrovirus infections allowed our distant ancestors to acquire the capacity to perform functions that are fundamental to human existence.

One remarkable example is a gene inherited from a retrovirus infection about 400 million years ago that plays a crucial role in memory formation. The gene does this by coding for tiny protein bubbles that help to move information between neurons, in a manner that is similar to the way viruses spread their genetic information from one cell to another. In the laboratory, mice that have this gene removed are unable to form memories.

Another mind-blowing example of a function that human ancestors acquired from retroviruses is the ability to give birth. When animals first evolved, they reproduced by laying eggs, and most creatures in the animal kingdom continue to give birth this way. Then, between 100 million and 200 million years ago, a shrew-like creature developed the capacity to gestate her young inside her own body – an extraordinary evolutionary advance because a foetus is much safer growing inside its mother’s body. It is only possible because of the placenta, a temporary organ that attaches to the uterus and allows nutrients and oxygen to pass from mother to baby, and carbon dioxide and waste to travel in the other direction, without provoking a devastating response from the mother’s immune system.

There is nothing like this interface between the placenta and womb anywhere else in our bodies. When geneticists looked at the gene responsible for creating it, they realized that it was almost identical to those used by retroviruses to produce the proteins that attach to cells they are infecting without triggering an immune response.The scientists concluded that a crucial function of the placenta didn’t emerge gradually as a result of evolution by natural selection but was suddenly acquired when a retrovirus inserted its DNA into our ancestor’s genome. So if one of our distant ancestors hadn’t been infected by this tiny virus hundreds of millions of years ago, humans would reproduce by laying eggs.