“So if we say a million, we mean a million pieces of DNA that we think belong to different organisms and among them represent different species,” Dr. Locey said.

The second approach was to use a well-known model of biodiversity as a basis for making predictions. “If you know the number of individuals, you can predict the most abundant species,” Dr. Locey said.

”So we used those two inputs — the number of individuals over all and the number of individuals belonging to the most abundant species — to estimate the total number of species.”

The two methods provided numbers that matched: Earth contains 10^11 to 10^12 species of microbes.

“We think our approach was rigorous in that we used a theoretical prediction and a statistical estimate,” he said. “We ended up with intersecting predictions based on different methods.”

Finding the number of species has broader implications, Dr. Locey said. “How many species could have actually evolved in four billion years? What are the upper constraints of evolution on Earth? How many species have evolved, how many could have evolved?