In a research summary posted on The Conversation, scientists attempted to recreate the chemical equilibrium of the cooling magmatic ocean which covered the planet billions of years ago.

The Earth arose out of a mechanism in which small objects snap to create larger and larger bodies. Using such a mechanism in the history of the earth would have enveloped our planet in a magmatic ocean; an ocean like this would have leaked into our planet the initial atmosphere, and a sufficient cooling to condense water vapor from the air to form condensed seas of water.

Venus never cooled down sufficiently to form water oceans. H2O in the atmosphere remained as water vapor and was lent to space steadily but inexorably. Earth became more favorable to life, whilst Venus became more and more inhospitable.

Henrik Hargitai, CC BY-SA 4.0, via Wikimedia Commons

Venus has an incredibly thick atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen, traced by other gasses, including sulfur dioxide. The atmosphere density of Venus is 93 times that of Earth, while the surface pressure is approximally 92 times that of Earth – a pressure equal to that almost 1 km (5⁄8 mi) deep below the Earth's oceans.

This atmosphere creates a heavy greenhouse effect in the solar energy system, producing a surface temperature of a minimum of 735 k (462 ° C; 864 ° F) and makes Venus' surface hotter than Mercury's.

By Brocken Inaglory - File:Venus with reflection.jpg, CC BY-SA 3.0

Astronomers have identified a cyclical fluctuation of the absorption of sunlight by the atmosphere. The axial tilting of Venus in minutes also minimizes the variation of a seasonal temperature. Studies suggest the atmosphere of Venus may have been much like the one surrounding earth, billions of years ago. Today, surface conditions are no longer suitable for any earthly life formed before the event. The likelihood of life in the top cloud layers of Venus 50 km (30 mi) up from the surface has been hypothesized, but the surrounding atmosphere is acidic.

According to the article, The Earth comes into being via a mechanism called the "accretion" in which tiny particles briefly clump together under the pull of gravity to form larger and larger bodies. The collision would have left Earth in a worldwide layer of rock called a "magma ocean" which would have poured into the first atmosphere of Planet hydrogen, sulfur, oxygen and nitrogen.

"If there's a lot of oxygen, it's bound in a 3:2-ratio to iron," researchers say, "We have multiple mantle samples, some of them carried to the surface by volcanic eruptions."

The early earth cooled down sufficiently to condense water vapor out of the atmosphere, creating seas of molten water like we have now. That will have left an atmosphere of 97% CO2 and 3% N.

Venus never cooled down sufficiently to form water oceans. H2O in the atmosphere remained as water vapor and was lent to space steadily but inexorably. On earth the seas have slowly but gradually washed down CO2 from the atmosphere. Earth was made more life-friendly while Venus grew even more inhospitable.