idk if you want a long answer, but its ketones
The earliest modern humans in Australia, dating to 50 - 40,000 yBP, were found at Lake Mungo. Early modern humans moved into North and South America because of population increase, competition for food and climate change. Modern humans likely migrated to the Americas via walking across the Being land bridge.
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xy is the answer of your question
George Monbiot is spearheading the Rewilding Movement. The environmentalist and writer is prominent in the platform. He was featured in TED Talk and talked about a large scale conservation approach aiming to restore the lost habitats. Here is an excerpt of his speech where he discusses the lost megafuna:
"What megafauna, you say? Well, every continent had one, apart from Antarctica. When Trafalgar Square in London was excavated, the river gravels there were found to be stuffed with the bones of hippopotamus, rhinos, elephants, hyenas, lions. Yes, ladies and gentlemen, there were lions in Trafalgar Square long before Nelson's Column was built. All these species lived here in the last interglacial period, when temperatures were pretty similar to our own. It's not climate, largely, which has got rid of the world's megafaunas. It's pressure from the human population hunting and destroying their habitats which has done so."
