For as long as there has been life on Earth, there has been extinction.It’s easy to assume that extinction must be a terrible thing, and that we should do everything we can to stop a species from disappearing.
We strive to avoid death throughout our lives, so we naturally see extinction as something to fight. But just as death will come to each of us, every species of animal, plant, fungus and microbe will eventually be consigned to the history books.
One million species across the planet are currently threatened with extinction. It’s a devastating statistic that instils an instant fight response. But as we place extinction under the microscope, we learn that it is a wholly natural biological process that actually helps to drive evolution. Species evolve; they adapt and they go extinct.
It’s a difficult and confusing narrative during a global ecological emergency, but if we are to stand any chance in prioritising species for conservation, saving vulnerable habitats and ecosystems, and ensuring we are not responsible for one of the most devastating episodes in the history of life on Earth, then we need to understand extinction itself.
What drives extinction?
So, what drives extinction, and what makes some species disappear more easily than others? We associate the word ‘extinction’ with two things. First, the dinosaurs, because they’re probably the group we most associate as having gone through the extinction process. There’s every chance a Triceratops is galloping through your thoughts right now.
Second, we often think that if something has gone extinct then maybe it was not quite up to the job, and possibly even deserved to disappear. This idea has been debated to explain the loss of the dinosaurs and even some of our own relatives, such as the Neanderthals.
Both are wrong. First, the dinosaurs are not the only group to have faced extinction (and in fact have never completely died out, but that’s another story). The trilobites, another prehistoric megastar group, was also lost through no fault of its own.
Comprising at least 20,000 known species that inhabited almost every type of marine habitat imaginable, the trilobites disappeared after a reign of more than 300 million years. It took the most devastating blow to life on Earth – the Permian mass extinction – to finally eradicate these armoured marine arthropods.
Second, going extinct is a natural part of the cycle of a species. It has nothing to do with how ‘good’ or ‘bad’ it is. The dinosaurs dominated the planet for more than 160 million years before the asteroid struck, and our prehistoric Neanderthal relatives thrived for several hundred thousand before disappearing in all but their genes.
And don’t think extinction simply picks off the weak and small. Ancient apex predators, such as the ratchet-jawed Dunkleosteus from the Devonian period, and megalodon, the largest predatory shark of all time, have also been claimed. Even T.rex, the ultimate Cretaceous predator, succumbed despite being perfectly suited to its environment.
When looking at extinction as a concept, we talk about being ‘extinct in the wild’, a straightforward term, and ‘functionally extinct’, which can refer to a situation where only one sex of a species is left, or when the last remaining individuals cannot reproduce. Fundamentally, the definition of extinct is the point when the last individual of a species or group dies.
Because extinction has been present since the first life forms on Earth popped into existence, then by default this must mean that an almost incomprehensible number of species have come and gone.
Research estimates that as many as 99 per cent of the species that have ever lived have gone extinct, which may translate to as many as five billion species exiting our planet. To be precise with this number is difficult, because even today, the estimated number of living species varies between 10 million and a trillion, with only 1.2 million so far documented.
A species typically has about 10 million years of evolving – eating, chasing, building nests and even going to the Moon – before it goes extinct. Some last for longer; some for less time. A few species of deep-sea fish, such as some members of the seven-gilled cow shark group, have been around for more than 60 million years; the unusual horsetails have remained largely unchanged since the early Jurassic period, some 170 million years ago.
A possible winner for the title ‘ultimate survivor’ may go to the horseshoe crab, which has remained in its current form for about 20 million years. Prior to that, it showed few differences for 150 million years and, as a group, stretches back an impressive 420 million years. At the other end of the scale, at least 500 species of fish in the East African Lake Victoria are thought to be less than 15,000 years old, and a very ‘new’ species of Galapagos finch only split from its cousins in 1981.
Why does extinction happen?
Every species evolves to fit a particular ecosystem or habitat and acts in a way that will help it survive and have young. Extinction happens when a species can no longer survive in its ecological niche. Lots of different things can cause this, and some are more natural than others. Some kill off a single species, others cause the loss of many at once, making extinction one of the most complex and important areas of study in science.
The dodo is a good example of a single species driven to extinction (when sailors arrived on its Mauritius island home at the turn of the 17th century); as are the passenger pigeon, thylacine, Steller’s sea cow and countless others. In contrast, the infamous asteroid wiped out 75 per cent of life on our planet in one go. Nothing larger than a sheep survived. Put simply, the common denominator for extinction is either when something happens too quickly or is too severe to overcome.
Imagine that a species of frog lives on one tiny island, and a volcano erupts, covering the frogs’ home with lava. If the eruption is sudden and the frogs have no time to escape and nowhere to hide, then it’s froggy finale. But if the eruption happens slowly and some individuals manage to flee to a rocky ledge or stream spared by the lava, the species will survive.
Not only will the frogs have escaped extinction, they could also be shaped by the experience. The phenomenally high selective pressures associated with extinction bring the opportunity for natural selection. The frogs that survived the catastrophe might have had particular traits or adaptations that gave them the slightest edge. Perhaps they could move more quickly, tolerate higher temperatures or hide more effectively. These characteristics are thus ‘selected’ for the next generation. (Of course, the opposite could happen, leaving the remaining population with a reduced gene pool.)
Can extinction be good?
Evolution isn’t directional, it doesn’t have a sense of purpose and it isn’t aiming for something. It’s a long line of random mutations that might either decrease or increase the chance of survival of that individual, and ultimately that species. But when a species, or group of species, goes extinct, nature does what it always does – it creates an opportunity for others. Nature hates a vacuum. It’s as if it can’t sit still, and needs to doodle or hum to pass the time.
When opportunities arise, ‘newcomer’ organisms don’t consciously think about conquering new environments or habitats. They develop if the right mutations come along at the right time, and when there is little or no competition.
Extinction, then, is ultimately responsible for the development of new species. This emergence of ‘new’ organisms is known as ‘speciation’ and with it, we often see evolutionary convergence, where unrelated groups overcome the same problems – flying, swimming or feeding – by different means, and analogous traits, where different anatomical structures perform similar roles across different species or groups, but are from different evolutionary lineages.
There are many examples of speciation where a ‘new’ group replaces another lost to extinction, but one of the clearest is when the large predatory marine reptiles disappeared at the end of the Cretaceous period, 66 million years ago.
With all these vertebrates suddenly gone from the oceans, a massive ecological gap was there for the taking. Within 10 or 15 million years, the first whales appeared in the fossil record, walking into the shallows and evolving into the behaviourally and physically diverse group of cetaceans we see today. Without that particular mass extinction, whales, dolphins and porpoises would probably have never existed. One group’s misfortune was another’s lucky day.
Extinction, then, can be argued as being the most creative evolutionary force in nature, helping to give rise to the dazzling biodiversity we see now and throughout the fossil record.
I’m not suggesting, though, that we step back and let the process run unchecked. Extinction is also a destructive force, and has the power to change the face of the planet. The entire story of life on Earth has been punctuated by catastrophic events of mass extinction, often over surprisingly short periods – such as the 11km-wide asteroid that hurtled towards Mexico at the end of the Cretaceous mass extinction.
While the loss of a single species rarely has a widespread cascade effect, the seismic shift in the diversity and accumulation of life brought about by a mass extinction can take tens of millions of years to recover from, leaving a scar on the timeline of life. As I write, approximately one million species are heading towards the cliff, such as the Hainan gibbon in China, with fewer than 30 individuals left.
Thanks to human-driven habitat destruction, pollution, introduced species, disease, hunting, climate change and various other influences, the rate and pattern of species decline and loss we are seeing now is as much as 10,000 times higher than you’d typically expect, which, based on evidence from the fossil record, usually sits at about one species per million per year. Our current rate of loss rivals some of the worst from previous mass extinctions and will be the first mass extinction event in the past 3.5 billion years that has been caused by a single species.
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When we peer back through the millions of species in the fossil record, we see an amazing variation stretching back for hundreds of millions of years. Whole groups have risen and fallen, to be succeeded by new biological pioneers. All the while, extinction waits like a spectre in the shadows, punctuating the timeline of each and every species. Every so often, these losses come together into world-changing episodes of widespread mass extinction.
Our best chance is to look back at these catastrophes and better understand the science of extinction – and then fight as hard as we can to avoid creating our very own Earth-changing event.
This is an extract from Dr Ben Garrod EXTINCT: The Story of Life on Earth, an eight-part book series
