Scientists can tell that Human beings (Homo Sapiens) also almost went extinct about 70,000 years ago. It is theorized that because of the Toba catastophe, the population of Humans was reduced to as few as 2000 individuals worldwide.
The Toba catastrophe is likely what caused Homo Erectus to finally go extinct after lasting for nearly 2 million years. They had a good run in those 2 million years, migrating from Africa to Asia, and as far North as Georgia.
Homo Sapiens also interbred with other species. Scientists studying Australian Aborigenes found that they have 5% DNA of Homo Denisova meaning that when Aborigenes trecked from Africa to Australia, they interbred with Homo Denisova.
European DNA is also 5% Neanderthal, meaning that the first Humans to arrive in Europe (Cro-Magnon) must have interbred with Homo Neandethelensis.
BTW, it turns out that Aborigenes were not the first Humans to arrive in Australia. An extinct sub species of Homo Sapiens has been found there, known as Mungo Man from 60,000 years ago.
http://en.wikipedia.org/wiki/Lake_Mungo_remains
It would have been great to meet some of these folks