Why is Australia full of poisonous animals?

The right ecosystem and random chance make Australia a promised land for animals that use venom as biological weapons.

The eastern brown snake ( Pseudonaja textilis ) hunts using both venom and constriction. (Photo: Kristian Bell)

Australia is home to a myriad of venomous creatures , including spiders, snakes, jellyfish, octopuses, ants, bees and even platypuses. Australia became a separate landmass about 100 million years ago when it broke away from the supercontinent Gondwana, according to Kevin Arbuckle, an associate professor of evolutionary science at Swansea University in the UK. Some venomous creatures simply got stuck in Australia when it became isolated , according to Live Science .

Many poisonous arthropods, such as the trap-jaw ant (genus Odontomachus), can inflict painful bites, but they also live in tropical and subtropical regions around the world, not just Australia. Similarly, the Australian bulldog ant (genus Myrmecia) , which can sting and bite at the same time, is among the world's most dangerous ants, killing three people since 1936, according to Guinness World Records. These poisonous ants existed in Gondwana at the time of the split and continued in Australia when it became its own continent.

As for spiders, funnel-web spiders (genera Hadronyche and Atrax) are unique to Australia and can kill humans with their venomous bites, according to Arbuckle. Male Sydney funnel-web spiders ( Atrax robustus ) have been responsible for 13 human deaths, although no fatalities have been recorded since the introduction of the venom serum in 1981, according to the Australian Museum. Another Australian widow spider, the redback ( Latrodectus hasselti ), can be deadly. Its ancestors also lived in Australia at the time of the continent's breakup.

Similarly, venomous cephalopods, including squid, octopus and cuttlefish, have been around for 300 million years. They lived in the surrounding seas long before Australia broke away from the continent of Gondwana.

Another reason Australia is so full of deadly creatures came 60 million years ago as a 'historical accident ,' says Michael Lee, professor of evolutionary biology at the South Australian Museum and Flinders University. Back then, continental drift pushed Australia toward the frigid Antarctic, wiping out most of its reptiles. As the continent slowly drifted north, it warmed up and attracted reptiles once again . By chance, 40 million years after this 'accident,' the first snakes to invade the continent came from the venomous viper family, which includes cobras, mambas, coral snakes and taipans. They became the ancestors of Australia's snakes, evolving into more venomous snakes.

Of Australia's 220 snake species, 145 are venomous. Deadly snakes make up 65% of the snake population here, although only about 15% of the world's snakes are venomous.

As for jellyfish, they're all venomous . They've been around for more than 500 million years, and have been floating through the oceans since before Australia existed. While box jellyfish ( Carukia barnesi ) and fire jellyfish ( Physalia physalis ) inhabit Australian waters, Arbuckle notes that they also occur in other tropical and subtropical waters. They're not a unique phenomenon. Instead, coastal Australia has an ecosystem that's well-suited to them.