One of the wondrous parts of biology is that it shows us that living beings – including ourselves – are not what we thought. We are now beginning to realise that ‘individuals’ aren’t particularly individual at all. The organisms of developmental biology, along with Darwin’s species, all turn out to be complex assemblages, typically made up of more cells of others than of their ‘own’.

Consider a phenomenon called microchimerism, which occurs when cells from a fetus pass through the placenta and take up residence in the mother’s body, and vice versa.¹ The cells of the mother infiltrate across the placenta into the child, and the cells of the child infiltrate into the mother. Thus each of us is a chimera of sorts, our bodies containing the cell lines of others.

If you are a first-born child, you will have a set of cells that come from your mother, including cells that she acquired from her own mother in the same way. If you are a youngest child, not only will you receive your mother’s cells, but you will also receive all of your siblings’ cells. We are thus not what we thought: every ‘I’ is also a ‘we’. And while the preceding example