The other "earth" is Venus and a European probe, called the Venus Express, is shedding light on how that planet, and ours, took differing paths. It all comes down to the "greenhouse gas effect". From The Guardian:
Scientists had puzzled over Venus's hellish characteristics. It is roughly the same size as Earth, with a roughly similar orbit. Both planets began life with similar atmospheres, but Venus underwent a ferocious greenhouse effect that left it with an atmosphere made up almost entirely of CO2 and almost no water.
"Earth and Venus were pretty much identical to start with ... It really made you think the physics was different, which obviously it couldn't be," said Prof Fredric Taylor at Oxford University. Some scientists had thought that Venus's proximity to the sun was key to its transformation, but the new data suggests a different explanation: "It's not evil, just unfortunate." Venus Express's data appear in a set of paper's in today's edition of Nature.
One finding is that the solar wind - a stream of charged particles from the sun - is stripping away water molecules from the atmosphere by breaking them into atoms of hydrogen and oxygen and blasting them into space. That cannot happen on Earth because its rotation creates a magnetic field that diverts the solar wind. Without water, CO2 in the atmosphere could not be laid down in oceans as carbonate rocks. "[On Earth] it's all in the white cliffs of Dover and places like that," said Taylor. The CO2-rich atmosphere led to a runaway greenhouse effect.
Venus Express has also found some intriguing features of the atmosphere. One instrument detecting electromagnetic frequencies has observed the tell-tale signature of lightning, something some scientists did not think was possible. "It is like some sort of echo," said Dr Magda Delva at the space research unit at the Austrian academy of sciences in Graz. "If you have lightning then chemical reactions are possible that would not under normal conditions take place ... At least on Earth this was important for the beginning of life."