Culture and its transmission from generation to generation is the defining feature of humanity. It is perhaps the best candidate for the thing that separates us from other beasts. Though there are other species that have been shown to hand down accumulated knowledge – including chimps, who show some evidence for cultural transmission of tool-use – no other animal approaches our ability to layer breakthrough upon breakthrough in such a complex way, and certainly no other animal does it with the conscious intent to lift future communities beyond the achievements that came before. That is a human distinction if there ever were one.

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Why has no other animal evolved this demonstrably useful ability? There are lots of intelligent animal candidates, but most have some straightforward biological reality holding them back.

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Take the octopodes: famously intelligent, and with a rich visual communication system in their incredible instantaneous colour changes. But their very short lifespans, coupled with rapid senescence and death after a single mating and clutch of eggs, leaves little time for a parent to hand on knowledge to offspring. Most fish and reptiles face a similar challenge – though they do not die after reproducing like octopodes, they do tend to produce large clutches of eggs that hatch and mature without much, if any, parental input, relying on the statistics of very large clutch sizes to ensure a few offspring reach adulthood.

The last time I spoke to Godfrey-Smith about this, he asked if I had any ideas why birds hadn’t produced a highly cultural species. I didn’t then. But I do now.
 

His question set me cogitating about birds and what makes them different from humans. This was a good counter-exercise for me – I’ve written a book about how much birds and humans share, from long childhoods to powerful brains to monogamous parenting. But the question of why no species, not even among the fantastically complex and intelligent large parrots, has developed complex, human-like culture is especially compelling in the context of those similarities.

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To answer it, we need to stop and consider how natural selection works. Natural selection (and, by extension, evolution) is a force with no foresight. It responds to the challenges that a species is currently facing. It does not, and cannot, ‘see’ broad sunlit uplands on the horizon and move toward them. This is because selection is a game of elimination – it happens when individuals die without reproducing, or having reproduced less than their neighbours. In each generation, that which is unsuccessful is culled by natural selection, and that which is successful endures. Occasionally, a random mutation or fortuitous combination of genes produces a family offshoot that is even more successful than its ancestors and cousins, resulting in higher rates of reproduction, or longer lives (which allow more reproduction), or higher offspring survival (who in turn reproduce more). Even in these cases, though, selection is not a positive force; those individuals live on a planet of scarcity. Their success comes at the expense of others that were just scraping through before the new mutations emerged. Those others stop scraping through and die, thanks to the success of the new mutation-bearing family.

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Think of lifespan versus number of offspring produced per year as our two spatial dimensions

This is important, because it means that natural selection can affect only what currently exists, not what could exist. Even if what could exist would be better or more successful. The consequence of this is that most evolutionary change must happen as a result of a ‘push’ rather than a ‘pull’ – a species’ traits change because they are currently inadequate, and being eliminated by selection, rather than being pulled toward a better alternative.