Human-ish

In the fifth season of the BBC's Doctor Who, about a voyaging alien "Time Lord" with a soft spot for human companionship, the Doctor's companion is shocked to discover that he is extraterrestrial.

"But you look human," she says. He shakes his head indignantly: "No. You look Time Lord. We came first."

Long gone are the days of budget CGI aliens with huge, bulging skulls, antennae ears and leathery green skin. Most recent depictions of aliens look quite a lot like us. 

Now, some scientists think the media might have got it right. Last year, a meteorite crashed in the town of Winchcombe in Gloucestershire, England, and a new analysis of its components had interesting results.

Lab researchers' breakdowns of the 500g fragments of meteorite that made it to Winchcombe from Jupiter - which is approximately 649 million kilometres away - showed that 11% of its weight was made up of water.

Even more strangely, the water extracted from the meteorite had around the same ratio of hydrogen atoms as on Earth, meaning that it closely resembles the composition of the water that created life here.¹

The news seems to add more evidence to an existing theory: that Earth's water, the foundation of every organism that exists on our planet, was brought by asteroids. And if asteroids can carry water almost identical to ours across the universe, other planets could carry similar life to ours.

If alien life forms developed on a planet with a similar environment to Earth, they might appear "humanoid". Evolution could have pointed them in the same direction, giving them a similar appearance to us.

But there is a longstanding debate among biologists about whether evolution could ever create something close to "human" ever again.

Stephen Jay Gould famously theorised about "replaying the tape" - giving history a blank slate and seeing if evolution would repeat itself in the same way - and concluded "I doubt that anything like Homo sapiens would ever evolve again."²

But plenty of scientists disagree, pointing out that there is evidence that unrelated organisms evolve similar features due to similar environmental challenges.

This could mean that aliens living on different planets confronting similar challenges could look exactly like us, even if we were raised in totally different corners of the universe.

Could humans already be living on other planets? 

Yes: If we think about how big the universe is - possibly limitless - the law of probability states that there must be some human, or at least humanoid, species living on different planets. In fact, it is much more likely that we would find something with evolutionary similarities to us than something totally different.

No: Even if some small conditions were similar, small differences could create huge changes in the evolutionary process of life on other planets. It is highly unlikely that we would find anything out there that strongly resembled us. Besides, we do not even have proof of any extraterrestrial life yet.

Or... It is unlikely that "humans" could be living on other planets, because regardless of the similarities, it is functionally impossible that a lifeform absolutely identical to a human would have developed on a different planet. But there could certainly be similar lifeforms.