Our body parts are all quite different – in form as well as function. The roughly triangular shaped dark-brown coloured liver processes blood to make it nutrient-rich and also produces bile – a fluid vital for digestion. The skin on the other hand takes the form of the animal itself and acts as an interface between the animal and the environment. Each of the diverse body organs grows by multiplication of cells of specific type. It is therefore weird to even think that they all originate from one single cell. It is true however that we all started as a single cell – the zygote.
As if that was not weird enough, the zygotes of different vertebrate animals are not different at all if one ignores the genes inside. Even during the early development stages, it is difficult to distinguish between a human embryo and a fish embryo.
Like the zygote, sex is another common denominator of most animals and many plants. If all sexually reproducing organisms had feelings (and money) like humans, they would buy erotic stories more readily than science fiction. That’s the benefit of appealing to the lowest common denominator that many storytellers aspire to draw.
For marine life, ocean – specifically the top part which receives light from the Sun – is where it’s all happening. The equivalent of ocean on land is the soil. Plants directly grow on soil whereas animals roam around and eventually merge themselves with soil. Plants and animals, and also a few other allied life forms grouped under Eukaryotes, have a basic unit – cell – which is the building block of their entire structure. Scientists have come up with the concept of LECA – last eukaryotic common ancestor – to represent life that could potentially have been at the junction where the plant, animal and other eukaryotic branches meet down the evolutionary tree of life.
LECA however is not the lowest common denominator of cellular life because it does not represent bacteria and archaea which also are made of cells, albeit lacking a nucleus. So encompassing them all is another imaginary ancient life form christened last universal common ancestor or LUCA. LUCA is sometimes also expanded as the last universal cellular ancestor. If, leaving the functions aside, we deem the information as the real ‘life’ thing, we can arguably consider as the lowest common denominator the genetic code or even the molecules that make up the genes. Beware though, any such move will immediately bring the viruses within the scope of life.
Whatever is the real lowest common denominator of life, cell is still the universally agreed basic unit. Just a casual look at cell structure presented in a text book makes one marvel at the level of organisation inside. No wonder then that life forms are called organisms, a term etymologically related to ‘organisation’.