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Are we living through the sixth mass extinction of life in Earth history?

That we are is something you'll - and you'll easily find dozens of taking the notion at face value.

It's even acquired a posh name - the .

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, a team of researchers looks at all the evidence that's been assembled down the years and asks whether it's really happening.

The previous "Big Five" events all involved, as far as scientists can tell, a loss of at least 75% of species in existence at the time - 96% in the case of the 250 million years ago.

Are we really on a trajectory towards something that serious?

Answering that question means addressing a huge number of subsidiary ones - notably, how can we tell?

It's not a trivial matter.

Assessing current extinction rates is difficult enough given that less than 3% of the world's known species have been .

Also, what do we mean by "current"? If the Holocene epoch is taken as beginning about 10,000 years ago at the conclusion of the last Ice Age, what tools can we use to assess extinction rates back then?

The problems become even greater when you try to look back to the previous five mass extinctions.

The fossil record is pretty much our only friend here, despite advances in molecular phylogeny techniques.

And a patchy friend it is, given that only certain kinds of creatures are preserved in fossils, that only a fraction of the fossil beds existing in the Earth's crust are accessible for inspection, and that looking back to the earliest of the Big Five (the ) entails making sense of evidence laid down 440 million years ago.

Another question is whether you measure the speed - the extinction rate - or the number of species that ultimately disappeared - the magnitude.

The review concludes that the magnitude of extinctions isn't yet at Big Five levels - but that's only part of the story..

Despite all these caveats and more, the authors (led by of the University of California) do come to some conclusions, which you could summarise by saying that the amount of extinctions are not yet enough to make this classify as the sixth big one, but the rate of disappearance and the amount of stresses on the natural world suggest we are getting there:

"The recent loss of species is dramatic and serious but does not yet qualify as a mass extinction in the palaeontological sense of the Big Five. In historic times we have actually lost only a few per cent of assessed species...

"...current extinction rates are higher than those that caused Big Five extinctions in geological time; they could be severe enough to carry extinction magnitudes to the Big Five benchmark [of 75%] in as little as three centuries.

"It is encouraging that there is still much of the world’s biodiversity left to save, but daunting that doing so will require the reversal of many dire and escalating threats."

There are two big factors that separate the Holocene extinction from all the others.

Firstly, it is the only one that is being driven by the expansion of a single species - Homo sapiens.

Secondly, Homo sapiens is particularly attracted to saving things.

So whereas the cataclysmic ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods saw species become first rare and then extinct without any chance for salvation, now we are seeking where we can to keep the last remnants of many species alive, either in cordoned-off areas of natural habitat or in zoos.

On the scale suggested by the Nature review, this isn't a feasible intervention; it may feel good, but it's barely a sticking plaster. If it's correct, tackling those "dire and escalating threats" urgently is the only way to prevent the mass die-off reaching Ordovician proportions.

Otherwise, the Big Five will become the Big Six - driven by humanity's expansion, and achieved within a few human lifespans.

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