There were giant dogs roaming North America 50 millionyears, but they got killed off by the changing climate. Theywere bigger than today's wolves and had gigantic teeth thatcould crush bones. In fact, the diversity of all largemammals on Earth plummeted starting during the last Ice Age,which occurred around that time, and the extinctions arestill continuing today.
Lee Bowman writes that there were originally threesub-species of dogs, but only the third sub-family, Caninae,includes today's dogs, coyotes and wolves, survived andspread worldwide. The others might have gotten too big fortheir own good. In a stable environment, evolution favorslarger body size, which makes it easier to avoid predatorsand capture prey. But when the weather became harsh, the bigdogs couldn't survive by eating the smaller mammals thatwere the only ones left.
Humans affect species by hunting them and by clearing land.Paleobiologist Anthony Barnosky says, "Humans tend to impactthe bigger animals, with the smaller animals as collateraldamage. Climate change is just the opposite?it affects thelittle guy first and then, through them, the big guys. Humanactivities today, combined with climate change, probably aregoing to result in inevitable extinction of many more species."
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