Yet some words have evolved in the other direction. In the 1830s, people would have ‘lighted’ a candle; nowadays we’d talk of having lit one. Why did these irregular words outcompete popular ones? A group of biologists and linguists at the University of Pennsylvania reckon that rhyming might have had something to do with it. They noticed that in the mid-twentieth century, Americans started saying ‘dove’ instead of ‘dived’ as the past tense of ‘to dive’. Around the same time, newly popular cars were causing people to adopt words like ‘drive’ and ‘drove’. Similarly, people started using ‘lit’ and ‘quit’ instead of ‘lighted’ and ‘quitted’ during the period that ‘split’ became a popular way of saying you were going to leave.
There are two main ways that new words and stories can spread through a population. Either they pass down from generation-to-generation, perhaps picking up some variations along the way; this is 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 all dolphins in Shark Bay would go on to use ‘sponging’. Only around one in ten have picked up the technique.[32] Why hasn’t the behaviour spread further? Twenty years after biologists first observed sponging, a group of researchers used genetic data to show that the tactic was almost entirely the result of vertical transmission. Dolphins are famously social, but it seems that after one initial dolphin came up with the innovation, it only spread through their family line. Individuals who weren’t related to them kept on foraging sponge-free. In effect, this family of dolphins had created their own unique tradition.
According to ecologist Lucy Aplin, both vertical and horizontal transmission of culture can occur in the animal world. ‘It really depends on the species, and also on the behaviour being learned.’ She points out that the type of transmission can affect how widely new information spreads. ‘You might imagine in, say, dolphins, where most of the learning happens vertically, you end up with family-specific behaviours and it’s quite hard for behaviours to spread more widely through the population.’ In contrast, horizontal transmission can result in much faster adoption of innovations. Such transmission is common in species of birds like great tits. ‘Much of their social learning occurs horizontally,’ Aplin said, ‘with information gained by observing unrelated individuals in the winter-flocking period, rather than transmitted from parent to offspring.’[33]
For some animals, the difference between transmission types could prove crucial to survival. As humans alter the natural environment more and more, species that can efficiently transmit innovations will be better placed to adjust to the changes. ‘Evidence is increasingly showing that some species can show a high degree of behavioural flexibility in the face of changing environments,’ Aplin said. ‘As a result, they appear to be successful at coping with human-modified habitats and human-induced change.’
Efficient transmission is also helping organisms resist human change at the microscopic level. Several types of bacteria have picked up mutations that make them resistant to antibiotics. As well as spreading vertically when bacteria reproduce, these genetic mutations often pass horizontally within the same generation. Just as software developers might copy and paste code between files, bacteria can pick up snippets of genetic material from each other. In recent years, researchers have discovered that this horizontal transmission is contributing to the emergence of superbugs such as MRSA, as well as drug-resistant STIs.[34] As bacteria evolve, many common infections may eventually become untreatable. In 2018, for example, a man in the UK was diagnosed with so-called ‘super-gonorrhea’, which was resistant to all standard antibiotics. He’d picked up the infection in Asia, but the following year two more cases appeared in the UK, this time with links to Europe.[35] If researchers are to successfully track
