A popular model of evolution suggests that the chances of humans evolving on Earth were incredibly low. This also means that intelligent life, like humans, may be very rare in the universe. However, some scientists believe that the way life and Earth evolved together made humanlike intelligence more likely than we think.

One of the most influential theories supporting the idea that humanlike intelligence is extremely rare is called the hard-steps model. This idea suggests that life had to pass through several highly unlikely stages to reach the level of intelligence we see today. If this is true, then Earth was very lucky to experience all these stages, and intelligent life on other planets is extremely rare.

What is the hard-steps model?

Some scientists in the 20th century thought that humanlike intelligence was very rare outside Earth. In 1983, physicist Brandon Carter supported this idea by noting a "remarkable coincidence"—it took Earth around 5 billion years to produce humans, while the Sun’s total lifespan is about 10 billion years. He considered three possibilities: first, that intelligent life forms very quickly, in just a few million years; second, that it develops at a similar speed as on Earth, taking billions of years; and third, that it usually takes much longer, even trillions of years, meaning Earth was extremely lucky to develop humans in such a short time.

Carter rejected the first possibility because Earth’s history shows that life took much longer to develop. He also rejected the second possibility because it seemed too coincidental that the sun’s lifespan and the time needed for intelligence to develop were so close. Instead, he believed the third explanation: that humanlike intelligence usually takes much longer to evolve than the lifespan of a star, making it extremely rare.

To explain his idea, Carter introduced "hard steps," which are rare and difficult evolutionary changes that had to happen for humans to exist. According to him, these steps must meet two conditions. First, they must be essential—if these steps had not happened, humans would not be here today. Second, they must be extremely rare and unlikely, meaning they usually take a very long time, close to 10 billion years, to occur.

Do hard steps exist?

Physicists Frank Tipler and John Barrow supported Carter’s theory, arguing that if these evolutionary steps were truly rare, they would have happened only once in the history of life on Earth. Scientists have identified several possible hard steps, including:

  • The origin of life—the first living cells forming.
  • Oxygen-producing photosynthesis—Bacteria creating oxygen.
  • Eukaryotic cells—cells with a nucleus (which humans and animals have).
  • Multicellular animals—creatures made of many cells.
  • Humanlike intelligence—the ability to think, reason, and create.

Each of these steps appears to have happened only once in Earth’s history. For example, all eukaryotic life comes from a single branch on the tree of life, suggesting that eukaryotic cells evolved only once. This supports the idea that these evolutionary steps were highly unlikely.

However, not all scientists agree with this conclusion. Evolutionary biologist Geerat Vermeij argued that just because we see something happening only once in the fossil record doesn’t mean it didn’t happen multiple times. Some reasons why we might think these events were unique include:

  • Extinction—Some forms of life may have evolved but later disappeared.
  • Incomplete fossil records—Fossils may not have been preserved or discovered yet.
  • First-come advantage—Once one species evolved intelligence or other major traits, it may have outcompeted others, preventing them from evolving in the same way.
  • Environmental changes—Once an evolutionary step occurred in one lineage, it may have changed the environment, making it impossible for other species to evolve in the same way.

If these alternative explanations are correct, then these evolutionary changes may not have been hard steps after all.

Why didn’t intelligence evolve sooner?

Humanlike intelligence did not appear earlier because Earth’s environment was not suitable for most of its history. For 90% of the time, there wasn’t enough oxygen in the air for humans to survive. Even for half of Earth’s history, there wasn’t enough oxygen to support eukaryotic cells, which are the type of cells found in humans and animals. This means intelligence didn’t evolve sooner simply because the planet was not ready. It took billions of years of changes in the environment before complex life, and eventually humans could exist.

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