It is widely accepted that the first terrestrial animals evolved from marine species. Mammals, including humans, are a class of animals that evolved from terrestrial species.
The jump from sponges to humans is, of course, a long one, but many researchers believe that sponges are the most likely candidate for an "Animal Eve," referring to a single group of organisms that, through many stages of evolution, gave rise to all animals alive today.
ANALYSIS: Are Sponges the World's Most Successful Animals?
The latest research also helps to answer a chicken-and-egg-type question: Which came first, a lot of sponges, or an oxygenated ocean deep? The answer, at least according to Lenton and his team, is the former.
"The effects we predict suggest that the first animals, far from being a passive response to rising atmospheric oxygen, were the active agents that oxygenated the ocean around 600 million years ago," he said. "They created a world in which more complex animals could evolve, including our very distant ancestors."