Carbon dioxide (CO2) is often criticized as a harmful greenhouse gas. However, its presence in Earth’s early atmosphere was crucial for the development of life. About four billion years ago, the Sun was only about 70% as bright as it is today. During this time, large amounts of CO2 were necessary to raise the planet’s temperatures enough to support the first microbial life.

But there’s a twist. While CO2 was essential for early life, too much of it can hinder the evolution of complex organisms, particularly intelligent beings like us. Earth managed to develop mechanisms to reduce CO2 levels through carbon recycling, a process where carbon is pushed below the surface by tectonic plate movements. Without this process, we might not be here today.

Other rocky planets orbiting sun-like stars may not have been so fortunate. They might have retained thick, CO2-rich atmospheres, which could support microbial life but pose challenges for intelligent life.

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Humans rely on oxygen (O2) for survival and exhale CO2 as a waste product. Astrobiologist Eddie Schwieterman noted that high CO2 levels can be toxic to us. Therefore, our evolution couldn’t have occurred in an environment with excessive CO2. Instead, we find ourselves in a world with high O2 and low CO2 levels.

Earth’s unique conditions arose not just from its location but also from timing. The Sun has been gradually getting brighter over billions of years, requiring more greenhouse gases like CO2 to keep Earth warm enough for life. As the Sun brightens, CO2 has been absorbed into carbonate rocks due to geochemical weathering.

Many exoplanets in their habitable zones would require significantly higher CO2 levels than those found on Earth to sustain life—potentially thousands of times more. Such high CO2 concentrations would be toxic to large, oxygen-breathing life forms, including humans. Thus, while many exoplanets might be suitable for microbial life, they may not support complex life.

Planets orbiting red dwarf stars, which are the most common type in the universe, may have different atmospheric compositions that allow toxic gases to accumulate. This indicates that the number of planets capable of supporting technological civilizations may be smaller than we think.

When searching for extraterrestrial intelligent life, the concept of a habitable zone becomes critical. The habitable zone for microbial life is much broader than that for complex life, meaning that while many planets could host simple life, few may foster intelligent beings.

Schwieterman’s research suggests that the emergence of intelligent life is not just a matter of random evolutionary events but is also influenced by the relationship between a planet and its star. Therefore, the number of planets with intelligent species is likely much smaller than those with microbial life.

As for Earth, it exists near the inner edge of the “conservative” habitable zone, requiring only modest CO2 levels to stay warm. This current low CO2 phase is special, but it won’t last forever. As the Sun continues to brighten, Earth could eventually enter a runaway greenhouse effect similar to Venus.