Seven billion is a lot of anything, let alone people, but we're certainly not the largest population of life on Earth. Estimates put bacteria at about four quadrillion quadrillion individuals, which means not only are they, by far, the most populous species in the world, but they also outnumber all other animal populations combined.
Grow a little bit in stature and your numbers will dip significantly, but 10 billion billion ants is nothing to sniff at. Individual size doesn't really matter anyway when your population combined weighs 10 times that of humans. And how many chickens do we need to sustain ourselves? There are some 20 billion domestic chickens on Earth at any given time to supply worldwide demand for meat and eggs.
But which mammal rules the Earth? You could say that humans have a pretty good claim to the throne, especially as a good number of the top 10 most populous mammals on Earth just happen to be our domesticated friends - cows and sheep populations currently sit at around one billion each, and cats and dogs are around 500 million each. But what about rats and mice? Watch RiAus's A Week in Science above to find out if their claim beats ours.