known as ‘vertical transmission’. Alternatively, tales may blend
across communities in the same generation, in a process of
‘horizontal transmission’. Da Silva and Tehrani have found that both
types of transmission have influenced the spread of folktales, but for
the majority of stories, the vertical route was more important. In other
areas of life though, horizontal transmission can dominate. Creators
of computer programs often reuse existing lines of code, perhaps
because there’s a useful feature they need to include, or because
they want to save time. In evolutionary terms, this means that
computer code can ‘time travel’, with bits of old programs or
languages suddenly popping up in new ones.[29]
If sections of stories or computer code mix together within a single
generation, it becomes difficult to draw a neat evolutionary tree. If a
parent tells their child a traditional family story, then the child
incorporates parts of their friends’ family stories, the new tale
essentially fuses all these different branches of stories together. The
same problem is well known to biologists. Take the 2009 ‘swine flu’
pandemic. The outbreak started when genes from four viruses – a
bird flu virus, a human flu virus and two different swine flu strains –
jumbled together inside an infected pig in Mexico, creating a new
hybrid virus that then spread among humans.[30] One gene was
closely related to other human flu viruses; another was similar to
circulating bird flu strains; others were like swine viruses. And yet,
taken as a whole, this new flu virus wasn’t really like anything else.
Changes like these show the limitations of a simple tree metaphor.
Although Darwin’s tree of life captures many features of evolution,
the reality – with genes potentially passing within as well as between
generations – is more like a bizarre, unkempt hedge.[31]
The processes of horizontal and vertical transmission can make a
big difference to how traits spread through a population. In the
waters of Shark Bay, just off the coast of Western Australia, a
handful of bottlenose dolphins have started using tools to forage for
food. Marine biologists first noticed the behaviour in 1984; dolphins
were breaking off bits of marine sponge and wearing them as a
protective mask while they rummaged for fish in the seabed. But not