The timeline of the hard steps in Earth's past that research discussed in this article builds on, reanalyzes and augments. Not shown is the era about a billion years in the future where all life on Earth will become extinct. Credit: Daniel B. Mills
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What took so long for humans to appear on Earth? The Earth is 4.6 billion years old, and life began about 4 billion years ago, yet humans—the only intelligent, technological species we know of in the universe—have existed only for the last 200,000 years. Why didn't we come sooner? What factor(s) delayed our appearance? And what can life's timeline here say about the possibility of other technologically advanced lifeforms in the universe?

One hypothesis of our evolutionary history is the "hard steps" model. In 1983, the Australian physicist Brandon Carter hypothesized that the path to humanity required "successful passage through a number of intermediate steps," each of which was very improbable and difficult given the time that had been available. Originally, Carter saw only two steps—the origin of the genetic code, and "the final breakthrough in cerebral development."

Many researchers have since modified Carter's idea and proposed more than two steps, with the most popular formulation of the model today envisioning five: 1) the creation of single celled life ( "abiogenesis" ), 2) the appearance of photosynthesis that creates oxygen, 3) the emergence of eukaryotic cells from prokaryotic cells, 4) the emergence of complex life, such as multicellular animals, and 5) the rise of Homo sapiens with an established language.

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So the question arises if intelligent, technological animals like us developed on Earth only late in the time available, cutting it close. Would the same fate befall any advanced extraterrestrial life elsewhere in the galaxy? Might they have developed too late in their sun's timespan to get to Earth? The hard steps model thus predicts that technological species such as humans on Earth are exceedingly rare in the universe.

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