A long-standing suspicion seems to have been confirmed: mammals like us spent their first hundred million years in the dark, and only came out in daytime when the dinosaurs disappeared. It is the first time we have had a firm date for this change. The first mammals to truly embrace the daytime were simians: our ancestors.

The first mammals evolved over 100 million years ago, but most remained small while dinosaurs ruled. Many palaeontologists think early mammals were “nocturnal”, only coming out at night.

Nowadays many mammals are active in the day – “diurnal” – yet most have eyes and ears adapted to darkness. For instance, most mammals have a thin reflective layer at the back of the eye that helps them see in the dark, and which causes the “eyeshine” of cats caught in car headlights. This is thought to be a hangover from nocturnal ancestors. However, this idea is hard to test, because eyes don’t fossilise.

at Tel Aviv University in Israel and his colleagues compiled data on the activity habits of 2415 living mammals. They then aligned this with the mammal family tree and reconstructed the likely activity patterns of modern species’ extinct ancestors. For instance, if two closely-related mammals are nocturnal, their common ancestor probably was too.

Step into the light

Moar found daytime activity only appeared 65.8 million years ago – within a few hundred thousand years of the mass extinction 66 million years ago that killed all the dinosaurs, barring birds.

That supports the “nocturnal bottleneck hypothesis”: the dinosaur extinction opened up new opportunities for mammals, particularly daytime foraging.

Because the team used “the largest dataset on extant species yet published”, they could date when many mammal groups became diurnal, says at Midwestern University in Glendale, Arizona.

Cathemeral activity – both at night and in the day – started to appear first. Nobody knows why, but there is evidence that long before their final doom – so mammals may have emerged into the day gradually.

The first cathemeral species was a hoofed mammal, which was the ancestor of cows, deer and hippos, plus whales and dolphins. It probably ate plants, and daytime grazing may have helped it digest food by warming up its stomach bacteria.

Emerging from darkness

However, cathemerality seems to have been unstable, says co-author at University College London, UK. “You see high rates of transition out of that state, but not into it,” he says. “Perhaps… they were just starting to exploit the daytime niche.”

The first mammal group to become exclusively diurnal was the simian primates, the group that includes monkeys and apes, about 52.4 million years ago. This may be why they are the only mammals with a visual system adapted to daytime foraging. In particular, they can distinguish red and green, which may help them spot ripe and unripe fruit.

Only one simian group returned to the dark: the of South America, which have large eyes to cope.

Simians’ sunny lifestyle may have facilitated the emergence of social behaviour like ours, says Moar. “I think it’s very difficult to be social when you’re nocturnal, because it’s hard to communicate between the parts of the group.”

We pride ourselves on our big brains, but when it comes to figuring out whether people or other animals with particularly big brains do better than others, the evidence has been lacking.

Now, for the first time, a study in red deer is showing that bigger brained mammals tend to be more successful in the wild, and that brain size is a heritable trait that they can pass on to their offspring.

Corina Logan from the University of Cambridge and her team have looked at the skulls of 1314 red deer (Cervus elaphus) from the Isle of Rum. The complete life histories of the deer are well known thanks to the Isle of Rum Red Deer Project, which has been collecting data on the island for more than 40 years, spanning seven deer generations.

“This kind of study has not been conducted before because it requires long-term data from a large number of individuals,” says Logan.

Heritable heads

The team found that the ratio of skull volume to body size was highly heritable, explaining 63 per cent of variation between individuals.

Female deer with larger skulls lived significantly longer and raised more offspring, though it’s not clear yet why bigger brains are advantageous to females.

It might be that they allow them to switch to alternative resources in times of environmental stress, or that they are simply correlated with longer life cycles, rather than causing the longer lifespans. But no one knows yet.

Surprisingly, larger-brained male deer showed no such benefits.

This suggests there may be sex differences in the costs and benefits related to larger brains, says Logan. Any benefits of brain size for male lifespan and reproduction may be masked by other factors.

“In species where males have to fight with other males to get access to females, like deer, the fitness of males may have more to do with being strong enough to successfully compete for females than with other attributes,” says Daniel Sol Rueda from the Centre for Ecological Research and Forestry in Barcelona, Spain.

Slow developers

Larger brains may offer adaptability, ingenuity and a better chance of survival, but they also take longer to develop and require more energy to maintain. Previous laboratory experiments with guppies, for example, found that larger brains were associated with improved learning but fewer offspring.

However, the new study – the first one to look at the issue in the wild – found no evidence of downsides of big brains, such as breeding more slowly or taking longer to reach sexual maturity.

This means natural selection may favour increases in brain size in female deer for their beneficial effects on survival and reproduction.

Logan says it’s possible that large brains aren’t as costly as people think, or there may be other ways that the deer make up for the energetic investment, such as weaker immunity.

Read more: Longer ‘penis’ drives evolution of bigger brains in female fish

Farming fungus in underground chambers (Image: Alex Wild)

Leafcutter ants in the rainforests of South America beat us to the invention of farming by some 50 million years. Now it seems that their fungus crop has undergone the same genetic changes as human crops.

As people selectively bred new crop plants, they often inadvertently made changes to their genomes. Wheat, bananas, tobacco and strawberries are all polyploid – meaning they have three or more copies of each chromosome rather than the usual two.

Now, a team at Copenhagen University have discovered that leafcutter ant crops are the same. and colleagues compared the fungus farmed by leafcutter ants with fungus kept by their less-specialised relatives. The latter ants’ crop consistently had two copies of each chromosome, whereas leafcutter fungus was polyploid, with between five and seven different copies.

Fast-track farming

“Polyploidisation is the fastest way to make a domesticated crop,” says from New York University. It makes it larger and more robust because it increases the number of copies of each gene, producing more gene products like growth hormones and immune proteins.

Early humans would likely have favoured polyploid crops for their increased productivity, and the same may be true for ants.

“About 50 million years ago, fungus-growing ants gave up their lives as hunter-gatherers to become fungal farmers,” says Kooij. He thinks the leafcutters took it further by selecting the more productive, polyploid fungi and encouraging their growth.

This final step may have helped leafcutters to increase their colony size by several orders of magnitude, just as crop domestication helped human populations expand and grow. Primitive fungus-farmers have colonies of up to 2000 workers, while the leafcutters can number into the millions. These remarkably successful insects dominate the rainforest, with a single colony taking as much as 500 kilograms of plant material each year to feed their fungal crops.

“The results of our study provide yet another piece of the puzzle to explain how these ants have been so extremely successful,” says Kooij.

Tastier bananas

Polyploid species are often unable to reproduce sexually. This speeds domestication for both humans and ant farmers as it prevents a crop species outbreeding with wild relatives. A loss of sexual reproduction also means no seeds, which allowed us to make fruits such as bananas more palatable.

“Humans have made edible bananas, bigger sugarcane and strawberries,” says Meyer. “And we’re currently making new polyploids for bigger kiwi fruit and seedless watermelon.”

Polyploidy also has other benefits – the extra genes provide raw material for natural selection. The team suggest this has driven leafcutter fungus to evolve new genes to digest fresh plant material, and to produce specialised edible growths called staphylae that made it easier for the ants to harvest their crop.

Journal reference: Journal of Evolutionary Biology, DOI: 10.1111/jeb.12718