Sorry, I'm offering a different way of looking at the question.

Here's why:

Until 1965 and the revelations of Mariner 4, a biologically active Mars was presumed. Why? Because there was nothing known about conditions on Mars which precluded biology. Nothing. Conditions were known to be difficult for most terrestrial organisms, but not impossible for all of them, and by definition, terrestrial organisms don't fit Mars anyway.

Biology which arose on Mars would adapt to Martian conditions -- whatever they may be or however they may change. There is still no evidence whatsoever that precludes the presence of biology on or within Mars. None, nada, zilch.

In the early scientific era, the presence of biology was the default position regarding pretty much all celestial bodies. When evidence was adduced that precluded biology on certain celestial bodies (let's say stars, for example) then biology was no longer asserted as likely or possible on such bodies.

Just so when it came to planetary and satellite bodies. Biology was presumed until its presence could be reasonably precluded. There are no fevered swamps on Venus, for example. The Moon's surface has been shown to be sterile. So on.

In the case of Mars, the basic situation -- that there is no evidence to preclude presence of life on or within the planet -- hasn't changed for generations. What did change?

I say what changed was a political perspective following on the application of the Lunar Paradigm to Mars beginning in 1965. Even though we have come very far from that point (thank goodness) the issue is still one of the "discovery" of life on Mars.

While we await that "discovery", there is enormous fodder for dispute and argument, which we've seen decades of already, with no possible way of resolution. All "discoveries" to date have been disputed to death -- not because the discoveries are necessarily wrong, but because there is no way to positively confirm any of them. At least there is no way to do it that consensus will agree to.

The arguments and disputes become tiresome and unproductive. They exist to exist. So I ask what happens if we turn the question around? Instead of presuming a sterile Mars and insisting on incontrovertible proof of biology before accepting any evidence of it, what happens if we presume the presence of biology on Mars and require evidence that precludes its presence in order to assert Martian sterility?

The question of whether there is or isn't life on Mars is still open under those circumstances, but instead of demanding an absolute proof of biology before its presence can be accepted (and you know what will happen when that "proof" is finally adduced: dispute will rage for generations over whether biology on Mars is the result of contamination from our spacecraft!) demand evidence that precludes life. Until that evidence is forthcoming, I say we'd do better to presume the presence of biology.

Why is it such an "extraordinary" claim now, when it wasn't considered to be one prior to 1965?