We ate them all.
https://ourworldindata.org/wild-mammal-decline
A global population half the size of London contributed to many of the world’s largest mammals to extinction. The per capita impact of our hunter-gatherer ancestors was huge. ... Trace the footsteps of these tiny populations of the past and you will find extinction after extinction after extinction.
https://ourworldindata.org/quaternary-megafauna-extinction
Between 52,000 and 9,000 BCE, more than 178 species of the world’s largest mammals were killed off. These were mammals heavier than 44 kilograms, ranging from mammals the size of sheep to mammoths.
From the article:
"But one may well ask: Why did humans become smarter all of a sudden? What was the advantage of having a large brain that consumes so much energy? We demonstrate that these biological and cognitive changes correlate directly with the size of prey. To hunt small elusive animals humans had to become smarter, faster, more focused, more observant, and more collaborative. They had to develop new weapons for hunting from afar and learn how to track their prey."
Eating meat is often credited with boosting human intelligence, but it probably was the challenge of adapting to decreasing biodiversity and the need to hunt smarter that truly accelerated our cognitive development. Similarly, as we grapple with climate change and pivot towards plant-based diets, these new challenges may offer opportunities for our intelligence to evolve even further. ;)