Where Did All the Giant Animals Go? A Case That Is Still Open
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Most of the world's largest land animals disappeared within the last fifty thousand years, and the argument about whether people or climate killed them has run for over a century. The evidence supports both and not equally everywhere.
What was lost
Large land animals disappeared across most continents in a relatively short interval geologically. The Americas lost mammoths, mastodons, ground sloths the size of elephants, giant armoured relatives of armadillos, sabre-toothed cats, dire wolves, giant beavers and native horses and camels. Australia lost marsupials larger than any surviving one, enormous flightless birds and a lizard several metres long. Eurasia lost mammoths, woolly rhinoceroses, cave bears and giant deer. The losses fell overwhelmingly on the largest species and spared smaller ones, which is an unusual pattern for an extinction and is the central fact any explanation must account for. Africa lost proportionally far less, which is the other central fact.
The evidence on each side
Both explanations have genuine support and specific weaknesses:
- •Timing in Australia and the Americas follows human arrival closely
- •Africa, where humans and these animals coexisted longest, lost the least
- •Islands reached late by people lost their large animals late, matching arrival rather than climate
- •Climate changed drastically at the same period, with ice sheets retreating
- •Earlier climate swings of comparable magnitude produced no comparable extinction
- •Direct evidence of hunting exists and is sparse relative to the number of species lost
Why size mattered
Whichever cause dominated, the selectivity by body size has an explanation that both accounts can use. Large animals reproduce slowly, with long gestation, few offspring and late maturity, so a population reduced by any cause recovers slowly and a sustained small additional mortality can drive it down indefinitely. Large animals need large ranges and large quantities of food, so habitat fragmentation hits them first. They are conspicuous and worth hunting, delivering a great deal of meat per animal. And they had no prior experience of humans in the newly colonised continents, which is the standard explanation for why African species, which had evolved alongside hominins for millions of years, fared so much better.
The other explanations offered
The argument is usually presented as a choice between hunting and climate, and several further proposals have been made and deserve mention. Disease introduced by arriving humans or their animals was proposed in the 1990s, and it requires a pathogen able to infect many unrelated species, which is biologically implausible and for which no evidence exists. A comet or asteroid impact around twelve thousand nine hundred years ago has been argued for repeatedly on the basis of particles in sediments, and the claimed evidence has failed replication on several occasions while continuing to attract advocates. Habitat change driven by the loss of the largest species, cascading into others, is a contributing mechanism rather than an initial cause. Most researchers now favour a combination in which human pressure fell on populations already stressed by climate.
What the loss did to the world
Removing the largest animals changed the systems they were part of, and the consequences are being studied actively. Large herbivores disperse seeds over long distances, and several plants with large fruits have no surviving disperser, leaving fruits that fall and rot beneath the parent tree. Grazing and trampling maintained open habitats, and vegetation shifted after the animals went. Nutrients were carried across landscapes in animal bodies and dung, and modelling suggests that transport collapsed by a large factor. Methane production by large herbivores was a measurable component of the atmosphere. These findings support proposals to reintroduce large grazers to restore those processes, which is a contested programme with a substantial literature.
The takeaway
Most of the world's largest land animals disappeared within the last fifty thousand years, with losses falling overwhelmingly on the biggest species and Africa losing proportionally least. Timing follows human arrival closely in Australia, the Americas and on late-settled islands, while climate changed drastically over the same period. Slow reproduction, large ranges and no prior experience of humans explain the selectivity either way.