essay neuroscience
8 | New Scientist | 4 January 2020
THE tropics are the most biodiverse regions on Earth. Now there is evidence that they are also the main source of evolutionary innovation and diversity.
Complex animals have dominated Earth for 541 million years, a time span called the Phanerozoic eon. Huge numbers of species have evolved and gone extinct during this time, in a complex story that includes fish, giant reptiles and whales.
However, in the 1980s palaeontologist John Sepkoski analysed the overall pattern of evolution in the sea, where the fossil record is best. He concluded that marine evolutionary history could be broken down into three supergroups, which he called “great evolutionary faunas”.
The first group was dominated by trilobites, which resembled woodlice, and bristle worms; the second by shellfish-like creatures called brachiopods; and the third by molluscs, which have persisted
to the present day. Other animals like land mammals probably followed similar patterns, but their fossil record isn’t complete enough for us to know.
Now, by analysing nearly 18,300 marine genera from the Phanerozoic fossil record, Alexis Rojas-Briceno of Umeå University in Sweden and his colleagues have found that the evolution of complex marine life is best described using four great groups of fauna, not three (bioRxiv, doi.org/dg98).
The first supergroup existed between 541 and 494 million years ago, spanning the Cambrian explosion in which many animal groups first emerged. As in the original analysis, trilobites dominated.
The second supergroup, dubbed the Palaeozoic, lasted from 494 to 252 million years ago. Creatures with hard outer shells were now widespread, including brachiopods. This phase ended when the end- Permian extinction wiped out almost all complex life on Earth.
This matches Sepkoski’s analysis, but the new study splits his third great fauna into two. In this version, the third supergroup is called the Mesozoic. It began in the wake of the Permian extinction and ended 129 million years ago, in the middle of the dinosaur era. This time cephalopods, the group that includes squid and octopuses, were the rulers.
The fourth and final supergroup, the Cenozoic, is still dominant today. During this time, clams and snails have made up much of the diversity.
The team found that all four supergroups originated in the tropics, then expanded through the oceans. This is in line with the idea that the tropics are hotbeds of evolutionary innovation, which has been promoted by David Jablonski at the University of Chicago.
It may be that the warmer temperatures and ample sunlight ensure there is plenty of food to nourish a multitude of species. However, John Alroy at Macquarie University in Australia isn’t convinced. “I have always been sceptical about the out-of-the-tropics hypothesis, because they have never seriously dealt with sampling biases, as far as I know,” he says.
For Alroy, the problem is that more fossils are known from North America, Europe and Asia, whereas we have few from South America and Africa. “That’s the fundamental problem with this kind of analysis, the concentration of data in a part of the world that has moved significantly,” he says. ❚
DAYTIME exposure to bright light seems to help improve sleep and relieve some symptoms of dementia.
As we age, our eyes’ lenses grow cloudier, so less light reaches cells in the retina that connect to the brain’s master clock. It is even worse for older care home residents who rarely venture outside, and who are often exposed to light at night to help keep them safe.
They can experience insomnia and disrupted circadian rhythms, which may present as excessive daytime sleepiness, nocturnal wandering and sundowning: agitation and irritability during late afternoon and early evening.
Since many care home residents with dementia spend their daytimes seated in communal areas, Mariana Figueiro at the Lighting Research Center, New York, and her colleagues designed a light table that directs light upwards. These were installed in eight US care homes, along with light boxes and floor lamps, to deliver light bright and blue enough to activate the circadian system.
The tables were used during daytime hours by 46 residents with moderate to late-stage dementia
for four weeks. In another four- week period, they used a table with dimmer lights in a more yellow hue.
The bluer lighting significantly improved sleep quality, and reduced depressive symptoms and agitation, as assessed by the residents’ caregivers (Journal of Clinical Sleep Medicine, doi.org/dg8v).
“They also told us that they were eating better and they were more social,” says Figueiro. She suggests this means they are getting better sleep, which affects agitation and mood. ❚
Trilobites dominated the first great wave of marine life
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“ The lighting reduced depressive symptoms and agitation, and improved sleep quality”
Tropical waters are an evolutionary hotbed
Light therapy helps improve dementia symptoms