Overkill: did humans cause a mass extinction?

The most popular theory for the cause of this mass extinction was first put forward by American scientist Paul Martin, nearly 40 years ago.

Continued from "The great migration: how modern humans spread across the world"

He put it down to the arrival of Homo sapiens. In both America and Australia these mass mammal extinctions followed shortly after the arrival of the first humans. In Australia they began about 40,000 years ago, in the Americas about 13,000 years ago. According to Martin, because animals in these continents had never come across humans, they were vulnerable.

Read the diary of any explorer who encounters a natural habitat where no man has ever been, for example Charles Darwin on the Galapagos Islands, and you will find that they always comment on the lack of timidity of the wildlife. It is still like this today in the few parts of the world which have no humans living near them. So when the first human wanderers arrived, flint weapons, bows, arrows and spears in hand, the animals they came across were fearless. They may have looked on with curiosity at these half-hairy, two-legged apes clambering ashore, but chances are that the horses would have just munched on. Even the lions, provided they weren't too hungry, would probably have just fallen back to sleep. Thus they were easy prey for hunter-gathering man with his sharp spears – so much so that in less than a thousand years most of the big game had been slaughtered, and many species were on the verge of extinction.

The theory also explains why in North Africa, Europe and Asia many similar animals survived the presence of mankind. Animals here had evolved alongside human species for over 2 million years, and had grown used to their appetite for meat and hunting. The experience of their ancestors had evolved into a well-honed instinct that allowed them to survive in sufficient numbers, avoiding contact with humans by running away and hiding. This meant that the mass extinctions seen in Australia and the Americas simply never occurred. So, the theory goes, in just a few years Homo sapiens single-handedly deprived nearly half the world's landmasses of all their large creatures by hunting them to oblivion.

Recently this theory, called the Pleistocene Overkill, has itself come under attack. For example, it doesn't explain why some species not generally eaten by humans (for example, sloths) became extinct, while others that were hunted (such as bison) survived. Mass slaughter by humans also doesn't explain why beavers, bears and bison all became so small.

The best theory seems to be one that blends the arrival of humans with the effects of natural, cyclical climate change. It goes like this: when humans first arrived on the virgin continents of Australia and the Americas, they indeed found big game were easy prey. Many of the key predator species, such as lions, tigers and wolves, were killed off in massive numbers by the two-legged hunter-gatherers. At the same time, temperatures rose rapidly, causing the glaciers to melt and the seas to rise. What was once a rich American landscape of parkland trees and pastures gave way to huge stretches of arid inland savannahs with dried-up waterholes that turned into thick conifer forests near the much wetter coasts.

Because humans killed off so many of the larger, carnivorous predator species, the populations of these animals' prey – herbivores such as bison, deer, sloths, horses and camels – grew uncontrolled because there was nothing left to eat them. They became so numerous that there simply wasn't enough food to go around. Combined with the changes in vegetation caused by rapid climate change, the effect was catastrophic. Herbivores were wiped out in their millions through starvation because the landscape couldn't support them any more, and only small species which could endure long periods consuming little food and water survived.

The intensive grazing of these huge overpopulations also contributed to the effect of climate change, accelerating the transition from parkland to grassland, making the landscape even less suitable for supporting future generations of large animals.

How fragile are nature's ecosystems. Add a new bit of something over here (humans), and see them remove something else over there (lions and sabre-toothed tigers). Now throw in a bit of random climate change, and devastation sets in on a massive scale. The role of humans in the annihilation of the large herbivorous marsupials and placental mammals of Australia and the Americas between 40,000 and 12,000 years ago – at the beginning of the last second to midnight on the 24-hour clock of Earth history – was humanity's first big impact on the Earth's fragile, changing natural environment. It would not be the last.

people'It can last and it's terrifying'
people Emma Watson addresses celebrity nude photo leak
Arts and Entertainment
Katie Hopkins appearing on 'This Morning' after she purposefully put on 4 stone.
peopleKatie Hopkins breaks down in tears over weight gain challenge
Have you tried new the Independent Digital Edition apps?
ebooksAn unforgettable anthology of contemporary reportage
Life and Style
fashionModel of the moment shoots for first time with catwalk veteran
Alexis Sanchez, Radamel Falcao, Diego Costa and Mario Balotelli
footballRadamel Falcao and Diego Costa head record £835m influx
Arts and Entertainment
Olivia Colman topped the list of the 30 most influential females in broadcasting
Life and Style
techIf those brochure kitchens look a little too perfect to be true, well, that’s probably because they are
Arts and Entertainment
Danish director Lars von Trier
filmEnglish-language series with 'huge' international cast set for 2016
Life and Style
Kelly Brook
peopleA spokesperson said the support group was 'extremely disappointed'
Andy Murray celebrates a shot while playing Jo-Wilfried Tsonga
TennisWin sets up blockbuster US Open quarter-final against Djokovic
Arts and Entertainment
Hare’s a riddle: Kit Williams with the treasure linked to Masquerade
booksRiddling trilogy could net you $3m
Arts and Entertainment
Alex Kapranos of Franz Ferdinand performs live
music Pro-independence show to take place four days before vote
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

HR Generalist (standalone) - Tunbridge Wells - £32,000

£30000 - £32000 per annum: Ashdown Group: HR Generalist (standalone) - Tunbrid...

Year 3 Teacher Plymouth

£23500 - £40000 per annum: Randstad Education Plymouth: Year 3 Primary Teacher...

Junior Software Developer - Newcastle, Tyne & Wear - £30,000

£25000 - £30000 per annum + benefits: Ashdown Group: Junior Web Developer / J...

Systems Administrator (SharePoint) - Central London - £36,500

£35000 - £36500 per annum: Ashdown Group: Systems Administrator (SharePoint) -...

Day In a Page

'I’ll tell you what I would not serve - lamb and potatoes': US ambassador hits out at stodgy British food served at diplomatic dinners

'I’ll tell you what I would not serve - lamb and potatoes'

US ambassador hits out at stodgy British food
Radio Times female powerlist: A 'revolution' in TV gender roles

A 'revolution' in TV gender roles

Inside the Radio Times female powerlist
Endgame: James Frey's literary treasure hunt

James Frey's literary treasure hunt

Riddling trilogy could net you $3m
Fitbit: Because the tingle feels so good

Fitbit: Because the tingle feels so good

What David Sedaris learnt about the world from his fitness tracker
Saudis risk new Muslim division with proposal to move Mohamed’s tomb

Saudis risk new Muslim division with proposal to move Mohamed’s tomb

Second-holiest site in Islam attracts millions of pilgrims each year
Alexander Fury: The designer names to look for at fashion week this season

The big names to look for this fashion week

This week, designers begin to show their spring 2015 collections in New York
Will Self: 'I like Orwell's writing as much as the next talented mediocrity'

'I like Orwell's writing as much as the next talented mediocrity'

Will Self takes aim at Orwell's rules for writing plain English
Meet Afghanistan's middle-class paint-ballers

Meet Afghanistan's middle-class paint-ballers

Toy guns proving a popular diversion in a country flooded with the real thing
Al Pacino wows Venice

Al Pacino wows Venice

Ham among the brilliance as actor premieres two films at festival
Neil Lawson Baker interview: ‘I’ve gained so much from art. It’s only right to give something back’.

Neil Lawson Baker interview

‘I’ve gained so much from art. It’s only right to give something back’.
The other Mugabe who is lining up for the Zimbabwean presidency

The other Mugabe who is lining up for the Zimbabwean presidency

Wife of President Robert Mugabe appears to have her sights set on succeeding her husband
The model of a gadget launch: Cultivate an atmosphere of mystery and excitement to sell stuff people didn't realise they needed

The model for a gadget launch

Cultivate an atmosphere of mystery and excitement to sell stuff people didn't realise they needed
Alice Roberts: She's done pretty well, for a boffin without a beard

She's done pretty well, for a boffin without a beard

Alice Roberts talks about her new book on evolution - and why her early TV work drew flak from (mostly male) colleagues
Get well soon, Joan Rivers - an inspiration, whether she likes it or not

Get well soon, Joan Rivers

She is awful. But she's also wonderful, not in spite of but because of the fact she's forever saying appalling things, argues Ellen E Jones
Doctor Who Into the Dalek review: A classic sci-fi adventure with all the spectacle of a blockbuster

A fresh take on an old foe

Doctor Who Into the Dalek more than compensated for last week's nonsensical offering