Popular Mechanics (South Africa)
Uh-oh… New research on extinctions shows life doesn’t always find a way
FIVE MASS EXTINCTION EVENTS ARE generally credited with the state of life on Earth today, but new scientific evidence suggests Earth’s history may be marked by additional extinction events, as well as seemingly incidental population explosions. Jennifer Hoyal Cuthill, PhD, a data scientist at the University of Essex in England, and her colleagues used a machine-learning algorithm to chart shifts in the diversity of life over time and found that life doesn’t always rebound after extinction. This new research ‘goes against some of the traditional stories about evolution, which focus on mass extinctions and what happens immediately after them,’ she says.
Scientists often assert that mass extinction events make way for mass adaptive radiation events, periods when surviving species evolve and flourish. When the dinosaurs went extinct roughly 66 million years ago, for instance, the rest of life on Earth had broader access to food and other resources. Those species thrived, and eventually gave rise to humans. The logic follows, but the new research says mass extinction events are not the sole cause of adaptive radiation events, if they contribute to those occurrences at all.
To study the ebb and flow of biodiversity through time, an international team of researchers first selected more than a million data points representing the occurrence of 171 231 prehistoric species from the Paleobiology Database. This publicly available database is a hub of fossil records – including everything from big-ticket, famous finds plucked from the Burgess Shale to minor, individual T-Rex fossils – spanning the 540 million-year period from the Cambrian explosion to the present day.
The researchers then fed this data, which altogether illustrates a timeline of life on Earth, into a machinelearning algorithm that charted when and where these organisms appeared along that timeline. In other words, the scientists could attach spikes and dips in population diversity to specific moments in history.
The algorithm concluded that while the ‘big five’ mass extinctions are in the 95th percentile of most disruptive population change events, so are ‘seven additional mass extinctions, two combined mass extinction–radiation events, and 15 mass radiations,’ the researchers write in the journal Nature. The ‘big five’ should really be the ‘big 29’.
Only one of the ‘big five’ extinction events – the PermianTriassic extinction, that wiped out about 96 per cent of marine species and around three-quarters of all landdwelling species 250 million years ago – is followed by a mass radiation event. ‘There are many other examples [of mass radiations], such as the Cambrian explosion, that don’t seem to have been triggered by an extinction,’
Hoyal Cuthill says. In these scenarios, instead of an extinction event replacing one dominant species with another – dinosaurs making way for small mammals, and ultimately, humans – species evolve and thrive via new ecological niches.
The Cambrian explosion, which is responsible for almost all animal life on the planet today, saw species move into new ecosystems and develop new traits such as burrowing and predation. Around 180 million years later, at the start of the Carboniferous period, life once again blossomed without the influence of a preceding extinction event. That time, some organisms emerged from the oceans and began to conquer land to find new sources of food.
A mass extinction doesn’t appear to guarantee a subsequent mass radiation, and what’s more, it seems a big enough radiation event can, in some cases, hinder growth and induce other extinctions. A viral, invasive species – such as the wild hogs of the southern United States or the lionfish found in Florida – can harm their environment’s ecological diversity so much as to oppress some species and possibly wipe out others. The environment’s biological carrying capacity, if you will, breaks under the population’s resource demand.
By questioning the idea that extinctions and radiations must be causally linked, these researchers have introduced a new series of questions that others can continue to study – say, for instance, how scientists can tackle the extinction crisis Earth is currently grappling with.