A brief history of Earth – part 3
Brought forward 24 hours in order to mark Earth Day, this is the third (and possibly the last) part of my mini-series of posts looking at what we can and should learn about the fragility and contingency of our existence (i.e. the fact that we might so easily not be here to ponder the meaning of it), based on Episode 2 of the excellent Australia: The Time Traveller’s Guide. Parts 1 and 2 (both on Episode 1) having been published here last Thursday and Friday respectively.
Whereas Episode 1 concluded with the establishment of a wide variety of marine life in the Cambrian Period (the first major geochronological division of the Palaeozoic Era), Episode 2 covers the development of complex life through most of the Palaeozoic Era; culminating in the extinction of 90% of life on Earth at the end of the Permian Period (251 Ma BP); something that those who ridicule climate change “alarmists” as being misanthropic will be pleased to see that I can – and will – admit had absolutely nothing to do with human activity!
As such, Episode 2 covers the following:
Ordovician (488 to 444 Ma BP) – including the emergence of fish like organisms with (at least partial) exoskeletons (i.e ‘placoderm’ fossils found in the Simpson Desert).
Silurian (444 to 416 Ma BP) – including the emergence of the first animals to venture onto land and leave the trails and/or tracks in the sand (as preserved in the rocks of Kalbarri National Park in WA); and the emergence of the first complex, self-supporting, plants on land (as preserved in rocks at Yea in Victoria).
Devonian (416 to 359 Ma BP) – the period in which the variety of fish living in the oceans seems to have increased greatly (if fossil preservation is an accurate indicator).
Carboniferous (359 to 299 Ma BP) – the period in which a similar explosion of plant life appears to have happened on the land.
Permian (299 to 251 Ma BP) – In Australia, this period is synonymous with glaciation on land and business as usual in the oceans.
Anyone who is bemused by all these names and ages, may find Lionel’s Time Spiral useful. Alternatively, those who prefer things to be organised in Table format, may prefer this from the Geological Society of America (N.B. international stratigraphic nomenclature may be a can of worms you don’t really want to open!).
In all of this, possibly the most important event was the emergence of the first pioneer plants, which would probably have covered large amounts of otherwise bare rock. I am trying very hard to avoid attributing conscious decision-making processes to non-sentient life forms but, even if they did not “decide to try and get away from all those nasty sea creatures”, it is hard to avoid the conclusion that these plants took advantage of the fact that there was nothing else on the land or in the sky that could eat them. It is certainly logical to conclude that, in the absence of predation, plants would have for the first time turned the planet green. It is therefore believed that plant life thus rapidly increased the oxygen content of the atmosphere and gave rise to the presence of ozone; thus blocking out all that nasty ultra-violet light from the Sun.
In order to discuss the development of life within the Devonian Period, Professor Richard Smith visited the Bungle Bungle and Napier Ranges of Western Australia (Purnilulu and Windjana Gorge National Parks respectively), in order to demonstrate how:
1. Pre-existing rocks were by then being recycled (weathered, eroded, and deposited) in huge rivers that meandered backwards and forwards across wide open plains thereby, over time, depositing thick sequences of alternating layers of grit, gravel and pebbles; subsequently cemented together and then eroded again.
2. Coral reefs at least as big as the Great Barrier Reef fringed the Devonian landmass now known as the Kimberley; but eventually emerged from the sea as a result of gentle uplift and/or sea level change (i.e. undeformed and/or intact apart from subsequent weathering and erosion).
Somewhat more mundanely, Professor Smith also noted that placoderm fish fossils can be found in limestones across much of the northern part of WA. However, just when you thought the story was getting rather dull, sex seems to have been invented: The first heterosexual reproduction seems to have been underwater; maybe this explains why so many women like birthing pools… As if that is not weird enough, Smith then introduces us to Devonian-style lungfish (i.e fish with lungs) still living in freshwater creeks in eastern Australia – which he suggests evolved to cope with drought (a problem in this part of the world for almost 400 million years!)…
In the Carboniferous, everything seems to have got very big – plants and animals alike – as a result of the atmosphere being 50% oxygen. It is almost a chicken and egg conundrum – which came first – but I am sure there is an explanation. There was also, possibly, a day of reckoning… In the interim, amphibians became quite common and reptiles invented eggs (i.e. more deliberate, light-hearted, anthropomorphic nonsense). Whereas the Carboniferous Period’s bequest to providential posterity in the UK was coal, its main legacy in Australia is the presence of some very large lizards. But Australia has of course not been left impoverished, far from it, its coal however is of Permian age.
By the Permian Period, the party was well and truly over – for plants at least: Whilst there is evidence for glaciation found in South Australia, the sea life seemed to continue to be very abundant – but with much reduced biodiversity. This is analogous to the current situation in the waters in polar regions today – very large numbers of a modest number of species (i.e. as a result of higher levels of oxygen solubility in colder water – the reason bubbles form in water when you heat it up).
And so we reach the end of the Permian – the period that has given Australia 50% of the fossilised carbon it is currently pumping into the atmosphere approximately 1000 times faster than the carbon was originally removed from the biosphere by the process of sedimentation. Furthermore, as George Santayana would probably be keen for all to note, the mass extinction of 90% of all life on Earth that then occurred was caused by the sudden release of gases including carbon dioxide. Other culprits, it has to be said, were hydrogen sulphide and sulphur dioxide. However, whilst the latter two would quite easily have poisoned the atmosphere and cooled the planet, it is the CO2 that was primarily responsible for the ocean acidification that ensured the elimination of most sea life as well.
Today, at the end of the Carbon Age, human beings have caused CO2 to build up in the atmosphere ten times faster than it has done at any other time in the Earth’s history; and we are now witnessing ocean acidification at a similarly unprecedented rate: Thus, ecologists like Peter Sale (and many others) warn us that we are perilously close to the point at which impacts upon marine biodiversity will be sudden and permanent. Shellfish will be unable to extract calcium carbonate from the water and, if the acidification does not kill them, then, in the case of corals, the increasing temperature of the water will.
We have been protected from – and possibly blinded to – the damage we are doing to the planet as a consequence of the cooling effect of all our other forms of pollution. However, as would now seem to be becoming ever more obvious, increasing CO2 is by far the most important factor driving anthropogenic climate disruption (ACD) and, unless we decide that behaviour modification is necessary, we may well cause the Earth’s sixth mass extinction. Indeed, there is accumulating evidence that it is already underway: In geological terms, the current rate of biodiversity loss is already probably unprecedented. Just because we can barely measure it, does not mean it is insignificant.
I therefore believe that it is imperative that humanity acknowledges that business as usual is not a survivable option.
Recommended reading: Richard Fortey’s Life – An Unauthorised Biography (2009).