Yesterday we tried to place the Holocene-Anthropocene Mass Extinction in the context of Earth’s past mass extinctions. Listing the Holocene Extinction as the ‘Sixth Mass Extinction’ proves problematic for various reasons. Today we offer additional context: although a mass extinction is defined as the loss of >75% of (larger) species – this definition does not take into account absolute numbers of biodiversity loss. To do so requires understanding of the net evolution of biodiversity.
The evolution of biodiversity on Earth. Biodiversity seems to increase with time over the Phanerozoic Eon, the current eon, during which complex life has existed. It means we ourselves evolved in times of plenty, but also that we have a lot to lose around us. In fact, the Holocene-Anthropocene Extinction – if it were to escalate – could wipe out more species than any of the Earth’s previous mass extinctions. Creative commons image, based on the same Nature publication from 2005 as discussed in yesterday’s article. For original image view full PDF.
Life on Earth first appeared approximately 4.28 billion years ago, really rapidly after the formation of Earth itself (4.54Ga). Yet then the evolutionary process stayed dormant for a very long time, until the onset of the Phanerozoic, when complex life (including vertibrates) erupted with the ‘Cambrian Explosion’ (right after Snowball Earth). This suggests the Cambrian was a lush, green, flowering, fertile place . In stead, it’s better to imagine a fluorescent tube-lit office with very many drawing boards. The Cambrian was not the Paradise Period – but rather the Period of Creativity.
Instead, despite the hardship of sometimes hospitable geological and astronomical changes, the five or more mass extinctions and numerous lesser extinction events, overall biodiversity seems to do well with time, with the number of genera increasing about tenfold over the course of the Phanerozoic, following the Cambrian.
Permian lesson: count your blessings
Is this a rule of thumb? Is life destined to survive and -paradoxically- even benefit from mass extinctions? At first glance of the above graph a tempting hypothesis, but probably the answer is no: a series of extinction events from the end-Ordovician to the end-Permian did produce a net downward trend in the fossil record over a period of some 200 million years, and it’s not unthinkable that if the Permian-Triassic Mass Extinction would have escalated beyond extreme carbon cycle disruptions (flood basalts, coal fires), planet-wide wildfires, forest-killing fungal plagues, a plague of methane-producing bacteria, ocean-wide anoxia and large-scale mercury poisoning, complex life would have had to return to the Pre-Cambrian drawing boards.
Instead, all the known Kingdoms of Life managed to escape through the end-Permian bottleneck – sometimes you’re just lucky. And if you look at the biodiversity evolution over the last 200 million years of Earth’s history, starting with the Jurassic Period, our collective happiness seems to know no bounderies – especially if you consider this one party-pooping event, the end-Cretaceous extinction, was probably not ‘produced’ by the Earth system itself (as were previous mass extinctions), but by some extraterrestrial interference (not suggesting an asteroid is an intelligent object, just to be sure).
Humans are a by-product of the Garden of Eden (a by-product that became a plague)
Perhaps this is a good point to add a disclaimer to the above title: the word ‘now’ (in ‘the Garden of Eden is now‘, implying Earth’s biodiversity is at its peak) is to be interpreted in geological terms, so ‘relatively recent’ (but before modern man’s expansion drift started, overhunting, deforestation, monoculture, river dams and parking lots, pollution, etc).
To be a bit more specific, we’re talking about the Tertiary (the period after the end-Cretaceous Extinction), and especially its second half, the Neogene Period (see ‘N’ in the graph), officially from 23 to 2.6 million years ago. It includes the Miocene (from 23-5.3Ma) and the paleoclimatologically well-researched Pliocene (5.3-2.6Ma). To link this to the timeline of human evolution: the hominidae (great apes) evolved 15 million years ago, 7 million years ago the hominina separate from the ancestors of chimpansees, 3-2.8 million years ago: earliest hominins (earliest humans, bipedal) appear: Australopithecus & Homo habilis. In other words: interesting times, at least for those with an interest in Genealogy.
Simplified version of the Tree of Life, for a single human being (Ludwig Herzog von Württemberg (1568–1593)). Over the course of the Phanerozoic evolution of complex life on Earth, it could have been extended another 542 million years back in time.
Although we tend to think of the Period after the end-Cretaceous Mass Extinction as the ‘Age of Mammals’ (following the ‘Age of Reptiles’) the role of all tetrapod animals is relatively small in the bloom of life of the Tertiary, or anywhere else in the longer post-Triassic rise of global biodiversity.
In fact, the bloom of mammals (the earliest of which actually already appeared in the late-Triassic, ±225 million years ago) is perhaps not so much a royal succession after the disappearance of the dinosaurs, but perhaps better considered a mere by-product of the rise of the flowering plants – the main direct source of food for us, and for additional land-based animals we may like to supplement the human diet with.
Flowers define the Earthly Paradise. This is the ‘Age of the Angiosperms’
To compare the respective biodiversity estimates: currently there are 5,487 documented mammal species (still less than reptiles at 8,734 species or birds (living dinosaurs!) at 9,990 documented species). Flowering plants, or Angiospermae have an estimated biodiversity that ranges between 223,000 to 420,000 species, estimates that come from an interesting debate with no less than four different publications in the journal Taxon, that each go under the same intriguing title: How Many Species Of Seed Plant Are There? (Govaerts 2001; Thorne, 2002; Scotland & Worthly, 2003; Govaerts 2003). A more recent (2016) publication in Phytotaxa called ‘The number of known plants species in the world and its annual increase’ settles somewhere in the middle, with a total number of 295,383 known angiosperm species – considerably more than the 1,079 species of gymnosperms, the other, older group of seed-producing plants, that includes for instance pine trees and ginkgo. Flowering plants also outnumber mosses (11,000 species) and pteridophytes (ferns and horsetails, 12,000 species).
[*) Although all such plant biodiversity numbers pale in comparison with arguably the most impressive wonder of evolution, namely the diversity of insects (2.6 to 7.8 million species) and other arthropods, as documented in PNAS in 2015 – also here the hand of flowers can be seen: if we look at individual characteristics of both insect and flowering plant species, many show clear signs of co-evolution – pollinating insects as the most striking example.]
Flowers now cover the Earth, but one day the first one ever opened…
Flowering plants really did reshape the surface of the Earth. Not so much because they are the primary food source for animals, but because it is these organisms, this portion of our planets biodiversity, that truly, literally, covers it – the fields, the forests – only excluding the (coniferous) taiga as a major terrestrial biome exception.
Still, this image of a flowering, and fruit-baring Earth, that apart from the 22,750 different species of the Asteracea family (asters, daisies, sunflowers) and the 21,950 species of the orchid family includes for instance 10,035 different grasses (think of bread, think of dairy), 19,400 different bean species and all of the world’s broad-leaved trees, is a really recent one.
Although the ancestors of the angiosperms diverged from the gymnosperms in the Triassic Period, it was not until the Jurassic that the first flower appeared and not until the Cretaceous that their diversity started rapidly increasing, replacing conifers as the world’s dominant trees perhaps as late as 60 million years ago.
Ours is the time of the flowering plants. Shown are 12 species from the Asteracea family. CC BY-SA 3.0
All this is to emphasize that in the history of Earth, the Garden of Eden is not some time long ago in distant geological history, the Garden of Eden is Now. That’s the Lamb We bring to the Slaughter – with the Holocene-Anthropocene Mass Extinction: biodiversity was never this high – which means an extinction wave could never before kill this many species.
How many precisely? That we’ll find out in part 3 of this series, tomorrow.
© Rolf Schuttenhelm | www.bitsofscience.org