The Cambrian revolution
Last post, I wrote about the early history of life on the planet, taking us
from its origins about 3.8 billion years ago, through the evolution of complex cells
(the progenitors of animals, plants, and all else too), and up to the brink of the Cambrian period,
which began about 570 million years ago. As I noted in my last post, although the most
significant evolutionary developments happened during the first three-odd billion years of the
history of terrestrial life, by layman’s standards, throughout all of this time, things would have
looked rather dull: until about 600,000,000 years ago for example, there were no large, complex
animals, and the entire land surface was still devoid of any life at all, which was confined
exclusively to various bodies of water.
Precambrian life forms: About 1500 million years ago alongside the earlier bacteria, there
appeared buoyant, single-celled creatures commonly known as acritarcs. The size of acritarc
fossils varies 10-100 µm. Some acritarcs have been placed together with green algae but the
majority remain unclassified. It has been possible to discern only a few species from the
Precambrian era. Picture: Tom Björklund
Source: http://www.fmnh.helsinki.fi/virtualexhibitions/historyoflife/acritarcs.htm
Still, it was during this period that life, which had started out as simple self-replicating
molecules, moved on to develop a cellular structure (the eukaryotes), acquired a nucleus (the
prokaryotes), forged a symbiotic relationship either with ærobic bacteria (which were the
forerunners of present-day mitochondrion), or with photosynthesizing prokaryotes (the ancestors
of the chloroplasts that we find in all plant cells today), and finally, about 2.1 billion years ago (a
date based on the earliest fossils of this type that we have found - those of Grypania), developed
into simple, soft-bodied, askeletal, multi-cellular forms (which go under the collective label of the
Ediacaran fauna).
Grypania Spiralis
Source: http://www.ut.ee/BGGM/eluareng/grypania.jpg
Some of the earliest known multi-cellular animals, members of the Ediacaran fauna, from about
570 million years ago: Charnodiscus (large, orange sea pen); Ediacaria (three jellyfish on left),
Kimberella (tall, skinny jellyfish on right); Dickensonia (large, flat, segmented worm), Spriggina
(small, slender, green segmented worm); Tribrachidium (pinwheel-shaped, possibly echinoderm);
and Parvancorina (lavender arthropod). Image of a diorama from the National Museum of
Natural History, courtesy of the Smithsonian Institution.
Source: http://eands.caltech.edu/articles/LXX1/justbreathe.html
Cloudina.
Source: http://cas.bellarmine.edu/tietjen/Evolution/Cambrian%20Explosion.htm
Of these latterly developed creatures, some were akin to present-day sponges and such,
whilst others were tube-shaped (for example Cloudina & the various Sinotubulites), and quite
possibly representatives of earlier and ultimately unsuccessful variants of multi-cellular
organization – i.e. life-forms that did not lead to any of the surviving animal groups of today.
At the time of which I am writing, the outlines of the continents too would have been
absolutely unfamiliar to any human time-traveller: About 300 million years before the Cambrian
(i.e. about 900,000,000 years ago), due to continental drift (a topic I will cover in a future post),
all the continental land-masses had formed a great super-continent (Pangaea). Now, that is to say
about 800,000,000 years ago, this huge land mass was in the process of breaking up. What
exactly the evolutionary significance of this break up is not clear, but palaeontologists have no
doubt that it had a major significance for the direction that evolutionary forces subsequently took.
Source: http://www.platetectonics.com/book/images/Pangaea.gif
Another key factor is that the photosynthesizers (mostly blue-green algae or
cyanobacteria) which had evolved earlier (somewhere around 2.4 to 2 billion years ago) may
have, by about 800 million years, ago drawn so much carbon dioxide out of the air, to meet their
metabolic needs, that the planet was in danger of freezing over (since carbon dioxide is a major
greenhouse gas, and is involved in the mechanism that helps to attenuate radiative heat losses).
As a result, the Earth was beset by the severest ice age ever in its 4.5 billion year-long
history. Fortunately for us, and all other present-day life forms however, something happened to
slow down and balance the rate of carbon dioxide consumption of these early cyanobacteria. The
initial stocks of free oxygen that these cyanobacteria had produced (as a by-product of the
photosynthetic process) were used up in oxidizing the Earth’s crust – which was composed of
many reactive chemicals, mostly metals which had lain dormant up till then because prior to this
generation of oxygen (which is a highly reactive element) there were no compatible chemicals
with which the metals could react. Of the reactive but dormant chemicals, the chief constituent
was iron, which readily oxidized in the presence of all that free oxygen: In other words, for about
400,000,000 years, the Earth rusted!
A photo of the Martian surface. Mars is red because of iron oxide (rust).
Source: http://dragon.larc.nasa.gov/viscom/mars-red.jpg
Eventually however, all the free iron was oxidized, and from about 1.8 billion years ago
onwards, the free-oxygen-levels in the atmosphere rose dramatically. Somehow, this rise
eventually triggered the evolution of life-forms capable of metabolising oxygen, and converting it
back into carbon dioxide – a much more energetic reaction than that which fuelled the
photosynthesizers (which all converted carbon dioxide into oxygen and various carbon
compounds, with the help of sunlight). These new oxygen breathing life-forms also fed on the
photosynthesizers, whose levels declined steeply after about 680 million years ago. Thus, the
depletion of carbon dioxide which was threatening to deep-freeze the planet was balanced by the
growing numbers of oxygen-breathers. The stage was now set for the explosion of species which
characterizes the Cambrian period – the Cambrian revolution.
There is much debate about just how rapid this ‘explosion’really was, but within a ‘brief’
span of 80 million years (570 million years ago, to 490 million years ago), the Cambrian
revolution (so called because the first fossils from this period were found in the Cambrian
mountains in Wales) changed the entire course of life on the planet.
There is also some evidence that just before the start of the Cambrian period, some sort of
mass extinction, one of several which have punctuated the story of life on our planet, cleared
most of the slate of the preceding Ediacaran fauna, thus giving the new Cambrian forms a clear
field.
Whatever the truth of that, after billions of years of there being only very small life-forms,
almost overnight (in geological terms) an explosion of speciation occurred, and the seas were
suddenly aswarm with a dazzling array of creatures. Apparently, one of the most crucial
evolutionary innovations which made this sudden diversification possible was the hard skeleton.
This has double significance for us, because skeletons fossilize much more readily than soft body
parts. Consequently, we have a far richer fossil record from the Cambrian onwards, than we do of
the periods before it. This in turn means that our knowledge of the history of life from the
Cambrian onwards is much more thorough than our knowledge of the preceding chapters of that
story.
Initially, the fossil record indicates, the skeleton was an outer covering of some hard
material - an exoskeleton (much like those that crustaceans and molluscs have today). This
skeleton was necessary before organisms could get much bigger because large bodies need some
form of physical support. The exoskeleton also afforded a degree of protection to the organism, as
anyone who has struggled to crack open a crab claw can testify. Somewhat after the advent of the
exoskeleton, some organisms took a different route, and began encasing the skeleton within their
bodies (developing endoskeletons - like the one you have), relying on other strategies for their
protection.
As I noted earlier, in all likelihood, the availability of free oxygen, which is a very reactive
element, also played a major part in the development of large motile organisms, which needed
lots of energy to power themselves, and which, in their metabolisms, took advantage of the highly
energetic potential offered by oxygen, to meet their increasing energy needs.
The extinctions that had just preceded the onset of the Cambrian were also apparently a
crucial precursor to the evolution of complex life on Earth. Although we are not sure of the
details, it does appear that animals are capable of very rapid speciation whenever new habitats
appear (for example in Lake Tanganyika, which formed about five million years ago, to date more
than one hundred species of cichlid fishes have differentiated out of a single original ancestral
form). In the case of the Cambrian, the number of fossil taxa that we have found, at certain points
increases as a geometric progression with time (the series 2, 4, 8, 16, etc. is an example of a
geometric progression). From the fossil record, we also know that all sorts of new designs were
tried out - tubes, spines, shells, etc. Of these, the more successful innovations were continued, and
developed to suit the ever changing environmental conditions, while the unsuccessful ones simply
became extinct. Thus, during its eighty-odd million year span, the Cambrian period was beset by
five waves of major extinction, which culled the extant species, and eventually sent them en route
to become fish, amphibians, reptiles, mammals, and avians. Next post, I shall trace the
development of some of these forms, and deal with the invasion of the land.