If we trace our family tree backwards far enough, we go back not just generations, but species. We know that – assuming we are human – our biological parents are human too, otherwise every birth could in principle be regarded as a speciation event, and the word species would become almost redundant. We also know that our grandparents, great grandparents and great-great-grandparents were also human, by personal knowledge, photographic evidence, or ancestral memory. But what about our mega-great-grandparents, or giga-great-grandparents?
If we go back millions or billions of generations, we go back beyond our human ancestors, beyond our mammalian ancestors, and their ancestors the ancient fish, right back to their own cellular and pre-cellular ancestors. Go back far enough, and one goes back beyond the invention of sex – the division between female and male – and so the distinction between giga-great-grandmother and giga-great-grandfather. (By now, timescales for measuring generations are a bit hazier, and we are no longer talking about human-scale generation gaps.)
The notion that all living things are connected up in one big family tree – the biological 'Tree of Life' – is one of the biggest ideas in science. This is not least because it means that humans are part of an extended family of all living things – dissolving the traditionally fundamental (human) philosophical distinction between humanity and nature. Indeed, it means that we are just a rather small twig off the end of the mammalian branch of a much vaster tree of all life on Earth that includes not only animals but plants, fungi, slime moulds, bacteria, and so on.
The details of the exact structure of the base of the Tree of Life are not completely clear. Some have suggested that there could have been more than one tree of life – each a completely independent growth. Indeed, there may have been many separate false starts that never grew into whole trees, but only got as far as small shrubs, or individual grass stalks that only had a single lineage and then stopped. But whatever the historical actuality, the idea of the possibility of different trees of life – of different 'species' of trees or shrubs or grasses of life – is in principle plausible.
And having several different such 'plants of life' would imply the independent emergence of life on Earth several times. This may lead us to consider the possibility of other trees of life independently springing up elsewhere in the universe.
These alien trees of life would contain – and hence be as varied and exotic as – any number of alien creatures that we could care to speculate about. But the trees themselves might be quite different from ours. For all we know, their branches don't just diverge, but might also merge or intersect in unexpected ways. And if there may theoretically be more than one 'tree of life' on Earth, we can imagine that many kinds of alien 'plants of life' are in principle possible – not only alien trees, shrubs and grasses of life, but alien Vegetables of Life, Lawns of Life and Weeds of Life.
In other words, the possibility that alien life might have the same kind of genealogical structure as life on Earth does not diminish the variety of individual life-forms that might be available – certainly, relative to the types of life-form that are normally conceived of in science fiction (a context in which biological and even physical possibility is not necessarily a constraint). In fact, if anything it opens up further possibilities, as it invites us to imagine different – and even more alien – kinds of alien 'organism'. For example, one could imagine an alien 'tree of life' that was itself sentient, and whose different branches could personally remember their 'ancestral' past as we remember our childhood. Or, just as we have carnivorous plants on Earth, might we not have alien 'Carnivorous' Plants of Life that devour (or graze on) other alien plants of life?
If these possibilities sound remote to human experience, let us consider one further possibility, closer to home. Just say life on Earth originated not on Earth itself, but was 'seeded' from somewhere else – where some kind of organic material floated across space, and landed on our barren planet and sprouted into our now-familiar Tree of Life? In this situation, our Tree of Life on Earth would be just a branch of a larger, older tree of life originating somewhere else in the universe. In this sense, our true genetic heritage would be extraterrestrial. And our giga-great-grandparents would be aliens.
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