"The phrase Earth-like does not refer to a planet that necessarily resembles modern-day Earth at all," Olson said. "It's actually a very broad term that encompasses a broad variety of worlds. It includes hazy worlds like the Archean; it includes icy worlds like the 'snowball Earth' intervals; it includes anoxic worlds with exclusively microbial ecosystems; it includes worlds with complex and intelligent life; and it includes worlds that we haven't even seen yet."
That's helpful for scientists, she added, who need several models for what life on other worlds might look like.
In spite of their differences, each of these periods in Earth's history share at least one characteristic: chemical imbalances in their atmosphere. That's because biological activity produces substances that otherwise have no business coexisting, Catling said.
Take methane and oxygen: Placed together, these gases quickly react and destroy each other. But there's plenty of both on Earth, because living things keep making them.
"If you find a system in equilibrium, you've found something that's dead. Or something that's not alive," Catling said. "When we see something unusual, that's out of whack, it can be a sign of life."
People have talked about this idea since the 1960s, Catling said, but hadn't really quantified it up until now. For this paper, the scientists ran simulations using the known chemical contents of each atmosphere to see whether any telltale chemical disequilibriums existed.
The researchers found that during the Archean, when there was little oxygen, the coexistence of methane, nitrogen and carbon dioxide in the atmosphere (together with liquid water) would have been a sign that living things were hard at work.
"Large fluxes of each gas in the absence of biology is really difficult to explain," Olson said of the coexistence of carbon dioxide and methane.
In the mid-Proterozoic, as oxygen-producing microbes rose, the giveaway would be a combo of oxygen, nitrogen and liquid water. Even if the levels of atmospheric oxygen are too low to be detectable, scientists could look for ozone instead, Olson said. That's because ozone (composed of three oxygen atoms) is made by reactions involving biologically produced oxygen and it produces a very strong signal that could be detectable even at low levels.
In the Phanerozoic, which includes the present day, the biosignatures would be oxygen with nitrogen and water. (Oxygen levels here would far higher and much easier to detect than in the mid-Proterozoic.)