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MAY / JUNE 2022 25To 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 machine-
learning 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 Permian-
Triassic extinction, that wiped out about 96 per cent of
marine species and around three-quarters of all land-
dwelling 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.
EXTINCTION 101
How the ‘big five’
extinctions shaped the
trajectory of life on EarthORDOVICIAN-
SILURIAN
EXTINCTION:
~ 440 MILLION
YEARS AGO
Roughly 85 per cent
of all animal species
on Earth die, thanks
to an ice age that
deprived the oceans
of oxygen and
flooded them
with toxic metals.DEVONIAN
EXTINCTION:
~ 370 MILLION
YEARS AGO
Low-oxygen
oceanic ‘dead
zones’, volcanic
eruptions, and a
possible meteorite
impact are thought
to drive up to 80
per cent of life on
Earth to extinction.PERMIAN-
TRIASSIC
EXTINCTION:
~ 250 MILLION
YEARS AGO
Earth’s ‘Great
Dying’ kills nearly
96 per cent of marine
life and 75 per cent
of land animals,
courtesy of volcanic
eruptions and
global warming.TRIASSIC-
JURASSIC
EXTINCTION:
~ 210 MILLION
YEARS AGO
Volcanic eruptions
across Pangaea fill
the atmosphere
with CO 2 , triggering
a global warming
that wipes out
almost 80 per cent
of life on Earth.CRETACEOUS-
PALEOGENE
EXTINCTION:
~ 66 MILLION
YEARS AGO
A 10 km-wide
asteroid slams into
Mexico’s Yucatán
Peninsula, and the
resulting climate
crisis kills up to
80 per cent of all
species on Earth.12345