Organisms similar to those on Earth should appear on other Earth-like planets, many of which are more ancient than ours, says a study on evolutionary convergence of life.
This would imply that extra-terrestrials similar to humans in looks and intelligence should have evolved elsewhere, as against the green, weird aliens popularised by Hollywood.
Their absence so far is puzzling, according to evolutionary biologist, professor Simon Conway Morris at the University of Cambridge.
Maybe the aliens are hiding, as suggested by Arthur C Clarke or as Stephen Baxter said, "We live in a virtual world. I don't honestly know. My suspicion is we have only begun to scratch at the surface of reality, for want of a better word."
Convergence is not just common, but everywhere, and it governs every aspect of life's development on Earth.
Proteins, eyes, limbs, intelligence, tool-making and our capacity to experience orgasms are inevitable once life emerges, argues Morris.
Predators like sharks, pitcher plants, mangroves and mushrooms would have evolved on Earth-like planets. Intelligence too is an inevitable consequence of this evolution, he says.
In his new book The Runes of Evolution, Morris builds the case for an ubiquitous map of life based on convergent evolution. The theory supposes that different species will independently evolve similar features. Evolution proceeds by a set of rules and is not random.
"Often, research into convergence is accompanied by exclamations of surprise, describing it as uncanny, remarkable and astonishing. In fact it is everywhere, and that is a remarkable indication that evolution is far from a random process. And if the outcomes of evolution are at least broadly predictable, then what applies on Earth will apply across the Milky Way, and beyond," Morris added.
Different species will evolve similar solutions to problems via different paths like the way the octopus has evolved a camera eye that is similar to that of humans while also being distinct in its own ways.
Their common ancestor lived 500 million years ago, suggesting the eyes must have evolved independently.
Similarly, collagen, the connective tissue protein in fungi and bacteria evolved independently. They do not appear alike but are very similar.
Life will evolve wherever it can, says Morris, especially intelligent life, as long as the worlds are not boiling or freezing, while referring to the numerous Earth-like planets discovered.
This statement is in contrast to a Pennsylvania University study that concluded extraterrestrial intelligent life in the universe either does not exist or human perceptions of advanced civilisations are deeply flawed, after it examined almost 100,000 nearby large galaxies for signs of highly advanced technological civilisations.