12th February 1832: There has been a little swell on the sea to day, & I have been very uncomfortable Charles Darwin’s diary entry on his birthday 190 years ago.
While on board the voyage of the Beagle, Charles Darwin recorded the journey and mentions notable days like Christmas day and New Year’s day, but although he records each year what he did on the 12th February, he never mentions that it was his birthday.
We like many others around the world like to mark the occasion for Darwin Day and this blog will take a look at the advancement in our knowledge around evolution during the 365 days since Darwin’s last birthday.
‘Good heavens, what insect can suck it!’
In 2021, A Madagascan hawkmoth, whose existence was predicted by Darwin and Wallace, was recognised as a new species. Known as Wallace’s sphinx moth, the moth is famous for its enormous tongue – the longest of any insect – that uniquely can reach the bottom of the nectar tubes of the Madagascan star orchid. In 1862, when Charles Darwin was sent a specimen of orchid from Madagascar with its incredible nectar tube measuring a full 30 centimetres long, he exclaimed in a letter to a friend: ‘Good heavens, what insect can suck it!’ Darwin speculated it would take a moth with an extraordinarily long tongue to reach the nectar that fills the bottom of these tubes.
Evolving together over time, the moth and the star orchid have influenced each other’s biology in a unique but still one-sided pollinator and plant co-evolutionary relationship. The tongue of the moth increased in length in step with the long nectar tube of the orchid until, eventually, the orchid became wholly reliant on the pollination services of the moths. The moths, however, are still able to pollinate other plants which also have long nectar tubes. The Malagasy moth was named Xanthopan morgani praedicta, meaning ‘predicted moth’.
Evolving a bigger brain isn’t always about intelligence
While The Origin of Species is possibly the most well known book written by Charles Darwin, he also published The Descent of Man (1871) in which he speculated on the evolution of the human brain in comparison with other animals. In April, a team of researchers studying 1,400 living and extinct mammal brains published new findings in Science Advances which challenges the notion that cognition is the primary driver of brain size evolution in mammals.
Over nearly 200 million years of mammal evolution, the researchers have shown that brain size is actually secondary to body size.
Prof. Anjali Goswami, a Research Leader ‘A lot of the time where it looks like brain size in increasing, it’s actually not that brains are getting bigger, but that evolution is acting to decrease body size. When researchers talk about brain size what we usually mean is relative brain size, which takes into account the relationship between your body and the brain. For example, a blue whale has a much bigger brain than humans, but it’s much smaller than ours would be if you blew a human up to the size of a blue whale without changing our proportions. In this study, we developed a method that discriminated brain size from body size and looked at how both are evolving separately. This gives us an entirely new, and surprising, picture of how mammals evolve big brains.’
The study showed that most changes in brain size occurred after two cataclysmic events in Earth’s history: the Cretaceous–Paleogene mass extinction event 66 million years ago, and a climatic transition 23-33 million years ago. After the mass extinction event at the end of the Cretaceous period, the researchers noticed a dramatic shift in brain and body size in lineages such as rodents, bats and carnivores as animals filled the empty niches left by the now extinct dinosaurs. Roughly 30 million years later, a cooling climate in the Late Paleogene led to more profound changes, with seals, bears, whales and primates all undergoing evolutionary shifts in their brain and body size. This is what has led to the pattern in brain sizes that we see in modern mammals today.
Not feeding as a tadpole speeds up frog evolution
In the biggest study of frog skulls to date, researchers have revealed that a frog’s environment is the main driver of its skull shape, but that tadpole feeding behaviour influences how fast frogs evolve.
There are more than 7,300 species of frog found all over the world. Frogs can live in water, in trees, under the ground and on top of the ground. There are two distinctive stages in a frog’s life. The first is its larval form, which can look like eggs or tadpoles, before they change into their adult form through a process called metamorphosis.
The frog skull shape is driven by where they live as adults, so if they live in trees or in the water, that has a big impact on what their skulls look like. Researchers found that while skull shape isn’t impacted by whether or not they feed as tadpoles, every time they lose that feeding larval stage there is a burst in evolution and diversification. In different species of frog, the tadpole will either stay in their eggs for the entire larval stage and go through metamorphosis within the egg, or the larvae can hatch and feed themselves as free-swimming tadpoles. Feeding as an aquatic tadpole is quite different from feeding as an adult on land and requires different adaption. For frogs, it’s not so much what you eat, but when in your life you eat it that seems to matter for evolution. If the tadpole experiences functional pressures from feeding at that early stage in its life, it can limit how much the adult can evolve. Darwin would surely have been delighted at the new research that links back to his original ideas. He devoted a whole section of the Origin of Species to the differences between adult animals and their larvae, and speculated on how the adaptations of the larvae might influence the evolution of the adult form.
Darwin’s Digitisation
One of the reasons we love to share updates on Darwin to commemorate his birthday is because we know that you, our audience love to read about this. Blogs that we have published on Darwin’s collections, Digitising Darwin and on the cargoes Darwin sent back to the UK have had thousands of reads from countries around the world including the UK, Australia, the USA, and South America where Darwin made many discoveries. Over 2021 we have been able to add 341 invertebrate fossils and 15 mammal fossils to the Museum’s Data Portal. The mammals were scanned in 3D and released onto Sketchfab, a platform for 3D models on which these models have seen over 12,000 views. In addition, we digitally reconstructed the partial skeletons of Macrauchenia patachonica and Scelidotherium leptocephalum for the first time and released videos of these onto YouTube.
The video shows the parts of the skeleton which were found by Charles Darwin in life position. A reconstruction of how the animal would have looked is then superimposed on the skeleton, with Darwin included for scale.
The video starts by showing the casts of the skeleton that were made before it was removed from the slab of rock it was discovered in. This is then superimposed on a drawing of the skeleton from the publication first describing it (Owen 1838-1840). The casts then move into anatomical position and the original bones of the skeleton are shown in pink. Finally, a reconstruction of the animal as it would have been in life is revealed.
These models tell the story of how we go from individual bones – which might not look like much – to reconstructions with muscles, tendons, and skin on them. It highlights what these bones can reveal. These fossils, which represent the first discoveries of these long-extinct mammals, have been used to inform global scientists understanding of what these animals would have looked like.
If you want to stay up to date with the Darwin digitisation project check out the Museum’s website and make sure you are following @NHM_Digitise and @NHMfossilmammals on twitter.