From the Proterozoic to the Cretaceous eras
Have you read the preceding page, "A walk through the evolutionary garden"?
Eventually, some single-celled organisms began to join together in more organised structures, constituting the first steps in the creation of multicellular organisms such as ourselves. This occurred during the…
Proterozoic Era - meaning 'earlier life', just before the proliferation of complex life on Earth (2,500 – 542 Mya)
The stromatolitic and free-floating cyanobacteria whose beginnings we noted in the late Archean era eventually changed the nature of the earth’s oceans and atmosphere which were formerly both highly anoxic and dominated by N2, H2O, and CO2. At the risk of repetition, at first this free oxygen was quickly reabsorbed in chemical reactions, such as rusting that bound it to iron. (The presence of huge bands of rust from the early Proterozoic era is one of the reasons we know about the increase in free oxygen). Secondly, when this source of iron was exhausted about 2 billion years ago, free oxygen was being produced too fast to be soaked up in this way and it was also released into the atmosphere. By 2 billion years ago, free oxygen may have accounted for 3% of the gases in the atmosphere; in the last billion years, the level has risen to about 21%[1]. This has been referred to as the Great Oxygen Event, notably in the Smithsonian Natural History Museum Exhibition graphic depicted below from the Smithsonian Exhibition Life and the Oceans.:
[1] Christian, op cit, 112-3; Hubble, op cit.
Proterozoic Era - meaning 'earlier life', just before the proliferation of complex life on Earth (2,500 – 542 Mya)
The stromatolitic and free-floating cyanobacteria whose beginnings we noted in the late Archean era eventually changed the nature of the earth’s oceans and atmosphere which were formerly both highly anoxic and dominated by N2, H2O, and CO2. At the risk of repetition, at first this free oxygen was quickly reabsorbed in chemical reactions, such as rusting that bound it to iron. (The presence of huge bands of rust from the early Proterozoic era is one of the reasons we know about the increase in free oxygen). Secondly, when this source of iron was exhausted about 2 billion years ago, free oxygen was being produced too fast to be soaked up in this way and it was also released into the atmosphere. By 2 billion years ago, free oxygen may have accounted for 3% of the gases in the atmosphere; in the last billion years, the level has risen to about 21%[1]. This has been referred to as the Great Oxygen Event, notably in the Smithsonian Natural History Museum Exhibition graphic depicted below from the Smithsonian Exhibition Life and the Oceans.:
[1] Christian, op cit, 112-3; Hubble, op cit.
Here, fuelled directly by the sun was a new source of free energy that could be used to construct more complex life forms. At some point a membrane made of fatty molecules surrounded the reacting chemicals, isolating them from the outside environment creating a primitive cell. The life forms that dominated the earth until less than 2 billion years ago were these simple, single-celled organisms that lived in the sea called prokaryotes or cells without a nucleus. Isolating these chemicals behind a semi-permeable membrane allowed reactions to thrive and the DNA to float freely within the cell.
With increasing efficiency, the membrane allowed energy and nutrients to come in and waste to get out. Meanwhile, the genetic material inside the primitive cells replicated by dividing, the offspring being in fact clones of the parents, leading to fast diversification. Blue-green algae and many bacteria are prokaryotes, primitive cells where the genetic material, the DNA used in their reproduction, is bundled into a coil with a membrane separating it from the rest of the cell. For nearly 2.3 billion years, life consisted of these single-celled microbes alone, some organised in colonies. These primitive organisms “transformed the earth from a cratered, moonlike terrain of volcanic rocks into the fertile planet in which we make our home[1]”.
Originally, free oxygen was extremely damaging to simple organic materials which is why life could not have appeared in an oxygen rich atmosphere, but after some 2 billion years of evolution, life adjusted to the presence of the new pollutant which was now in plentiful supply, thanks to the collective efforts of the photosynthetic blue-green algae. The surplus oxygen was also instrumental in creating the ozone layer which protected the earth from harmful ultra violet rays making it easier for life to spread and survive.
These changes may explain the appearance of new life forms called eukaryotes (cells with nuclei) about 1.2 billion years ago. We have little understanding how this occurred, although we do know that it took close to 2 billion years. The most accepted view is that eukaryotes developed from symbiotic alliances between prokaryotes (cells without nuclei). For example, mitochondria, the modern cells little engine, are believed to have been a separate organism in the distant past that was either eaten or absorbed by another cell. In eukaryotes (the more sophisticated cells like the ones in our body) an isolated nucleus houses the genetic material. The difference between the new cells and the old prokaryotes in the fossil record has been described as being akin to the difference between the Wright Brothers’ Kitty Hawk flying machine and the appearance of the Concorde jet a week later[2].
Eukaryotes reproduce in different ways to prokaryotes. Whilst the latter effectively produced clones of themselves, eukaryotes merged the genetic material from two different parent individuals. The DNA from two adults merges randomly to produce new strands of DNA containing a mix of the genes of both parents. Accordingly, individuals vary more among eukaryotes than prokaryotes. This precursor to sexual reproduction gave natural selection a greater variety of bodies to choose from in each generation, the result being a veritable profusion of life forms. In this way sexual reproduction counts, along with the appearance of eukaryotic organisms, as one of three major turning points in the history of the earth[3].
From unicellular to multicellular organisms
After about 3 billion years after life’s first known traces, a transition took place during the Proterozoic Eon (2,500 to 542 mya) from unicellular to multicellular organisms. Sponges, the first multicellular organisms, appeared as early as 1.8 billion years ago. The first multi-celled organism in the fossil record is a red alga called Bangiomorpha pubescens, which lived about 1.2 billion years ago. The crucial transition from single-celled to multicelled organisms - from our amoeba-like ancestors to sponges – was again most probably linked to the increase in atmospheric oxygen levels we have already described.
The expansion from one cell to many allowed specialised organs, and was another critical milestone on the erratic path to the great diversity of life today. Multicellularity is in fact one of the principal thresholds in evolutionary history. As with the transition from prokaryotes to eukaryotes (again: from cells without nuclei to those with a nucleus), multicellular organisms possibly also evolved through symbiotic trial-and-error processes, as different kinds of unicellular organisms linked to each other (or consumed each other) and became pluralistic in form and function. It is still difficult to understand how the different types of DNA became incorporated into a single genome. An alternative explanation, the Colonial Theory, proposes that unicellular creatures grouped in colonies that slowly evolved into multicellular animals.
The first extensive fossil evidence of multicellular organisms dates from the Ediacaran era (635-542 mya)[4]. Fossils known as the Ediacaran fauna are more than half a billion years old, and have been found in sedimentary rocks worldwide. Although life forms in the Ediacaran period lacked hard parts that could form fossils, their soft bodies made impressions on the sediment, as did their trails, and as sediment hardened into rock, it preserved examples of both. The evidence seems to suggest that these animals did not burrow into the sediment but lived on top of it. Some resemble modern groups, including sea-pens, but we are unsure whether most Ediacarans are direct ancestors of living animals or “evolutionary dead ends".
With increasing efficiency, the membrane allowed energy and nutrients to come in and waste to get out. Meanwhile, the genetic material inside the primitive cells replicated by dividing, the offspring being in fact clones of the parents, leading to fast diversification. Blue-green algae and many bacteria are prokaryotes, primitive cells where the genetic material, the DNA used in their reproduction, is bundled into a coil with a membrane separating it from the rest of the cell. For nearly 2.3 billion years, life consisted of these single-celled microbes alone, some organised in colonies. These primitive organisms “transformed the earth from a cratered, moonlike terrain of volcanic rocks into the fertile planet in which we make our home[1]”.
Originally, free oxygen was extremely damaging to simple organic materials which is why life could not have appeared in an oxygen rich atmosphere, but after some 2 billion years of evolution, life adjusted to the presence of the new pollutant which was now in plentiful supply, thanks to the collective efforts of the photosynthetic blue-green algae. The surplus oxygen was also instrumental in creating the ozone layer which protected the earth from harmful ultra violet rays making it easier for life to spread and survive.
These changes may explain the appearance of new life forms called eukaryotes (cells with nuclei) about 1.2 billion years ago. We have little understanding how this occurred, although we do know that it took close to 2 billion years. The most accepted view is that eukaryotes developed from symbiotic alliances between prokaryotes (cells without nuclei). For example, mitochondria, the modern cells little engine, are believed to have been a separate organism in the distant past that was either eaten or absorbed by another cell. In eukaryotes (the more sophisticated cells like the ones in our body) an isolated nucleus houses the genetic material. The difference between the new cells and the old prokaryotes in the fossil record has been described as being akin to the difference between the Wright Brothers’ Kitty Hawk flying machine and the appearance of the Concorde jet a week later[2].
Eukaryotes reproduce in different ways to prokaryotes. Whilst the latter effectively produced clones of themselves, eukaryotes merged the genetic material from two different parent individuals. The DNA from two adults merges randomly to produce new strands of DNA containing a mix of the genes of both parents. Accordingly, individuals vary more among eukaryotes than prokaryotes. This precursor to sexual reproduction gave natural selection a greater variety of bodies to choose from in each generation, the result being a veritable profusion of life forms. In this way sexual reproduction counts, along with the appearance of eukaryotic organisms, as one of three major turning points in the history of the earth[3].
From unicellular to multicellular organisms
After about 3 billion years after life’s first known traces, a transition took place during the Proterozoic Eon (2,500 to 542 mya) from unicellular to multicellular organisms. Sponges, the first multicellular organisms, appeared as early as 1.8 billion years ago. The first multi-celled organism in the fossil record is a red alga called Bangiomorpha pubescens, which lived about 1.2 billion years ago. The crucial transition from single-celled to multicelled organisms - from our amoeba-like ancestors to sponges – was again most probably linked to the increase in atmospheric oxygen levels we have already described.
The expansion from one cell to many allowed specialised organs, and was another critical milestone on the erratic path to the great diversity of life today. Multicellularity is in fact one of the principal thresholds in evolutionary history. As with the transition from prokaryotes to eukaryotes (again: from cells without nuclei to those with a nucleus), multicellular organisms possibly also evolved through symbiotic trial-and-error processes, as different kinds of unicellular organisms linked to each other (or consumed each other) and became pluralistic in form and function. It is still difficult to understand how the different types of DNA became incorporated into a single genome. An alternative explanation, the Colonial Theory, proposes that unicellular creatures grouped in colonies that slowly evolved into multicellular animals.
The first extensive fossil evidence of multicellular organisms dates from the Ediacaran era (635-542 mya)[4]. Fossils known as the Ediacaran fauna are more than half a billion years old, and have been found in sedimentary rocks worldwide. Although life forms in the Ediacaran period lacked hard parts that could form fossils, their soft bodies made impressions on the sediment, as did their trails, and as sediment hardened into rock, it preserved examples of both. The evidence seems to suggest that these animals did not burrow into the sediment but lived on top of it. Some resemble modern groups, including sea-pens, but we are unsure whether most Ediacarans are direct ancestors of living animals or “evolutionary dead ends".
Bill Bryson tells an entertaining story about the discovery of these Ediacaran fossils by a young geologist named Reginald Sprigg in the Ediacarian Hills in the Flinders Ranges in South Australia 1946. He made various attempts to publicise his discovery (amounting to nothing less than the dawn of visible life on this planet) in noteworthy journals and international conferences but no one attached any significance to what he had to say. Nine years later in 1957, a schoolboy named John Mason made similar discoveries in the Charnwood Forest in the English Midlands which were immediately recognised by those qualified to express an opinion on the subject as pre-Cambrian[5]. Many years later Sprigg's findings were ultimately accorded their true worth.
The first animals with skeletons
The ongoing quest for, and subsequent discovery of, fossils in this pre-Cambrian period and their interpretation is encapsulated in an article by Rachel Wood in the Scientific American in June 2019.[6] Drawing on evidence of fossil remains in as far diverse places as Siberia, Namibia and Brazil, it appears that not only was there evidence that animals started evolving skeletons and building reefs earlier than traditionally thought, but that ecosystem models were being developed showing that Ediacaran animal communities shared many ecological traits with Cambrian ones.
Among these finds are the oldest known creatures with external and internal skeletons composed of mineralized tissue, so the co-called "explosion" had a far longer fuse than was previously recognized. This is the key, Wood says, to “understanding the astonishing burst of diversification that followed in the Cambrian, and only now, with the discoveries of the older Ediacaran fossils, are we starting to see the roots of the Cambrian explosion.[7]
Wood examines these developments in successive periods of time:
- The earliest known representatives of the Ediacaran biota dating to about 571 mya were dominated by soft-bodied creatures up to a metre in height or width, some being rooted to the floor to maximize surface area, suggesting that these animals absorbed nutrients directly from the surrounding water,
- Then, around 560 million years ago, the Ediacaran biota diversified to include mobile forms that inhabited shallow seas;
- And then, after around 550 million years ago, the oldest fossils preserving external and internal skeletons suddenly appeared in limestone rocks, possibly because of the need for protection from predators, these including one known as Cloudina up to about 70 millimeters long and "resembling a stack of ice cream cones" capable of actually cementing themselves to one another to form a reef.
This finding has established Cloudina as one of the oldest reef-building animals, pushing back the record of this way of life by some 20 million years - and then it was found that Cloudina persisted into the Cambrian.
One possible explanation for this development and diversification, Wood says, is the possible role of shifting oxygen availability and it may be, she speculates, that at some point in the time spanning the Ediacaran and Cambrian, oxygen levels may have risen beyond a certain critical threshold, allowing animals to flourish.[8] “Dissolved oxygen in the oceans probably reached a threshold or series of thresholds during the Ediacaran that allowed animals to diversify by meeting their increasing metabolic demands as they became more mobile and active”. Soft-bodied animals may have been only able to evolve their calcium carbonate skeletons once oxygen levels reached such a threshold, allowing formerly isolated oases to expand, connect and achieve stability on a global scale.
In other words, Wood concludes, fluctuating oxygen conditions may have created critical opportunities for evolutionary innovation in soft-bodied animals. This remains a minefield for considerable ongoing research.
[1] Christian, Maps of Time, 113 and footnote 6.
[2] Ibid, 114.
[3] Ibid, 117.
[4] So named, after the Ediacaran Hills of the Flinders Ranges, about 300 miles north of Adelaide in South Australia. For the ongoing search for fossils in this area, see the South Australian Museum's website at http://www.samuseum.sa.gov.au/research/palaeontology/fossils
[5] Recounted in David Attenborough's First Life DVD.
[6] Rachel A Wood, paleontologist and geologist, University of Edinburgh, "The rise of animals", Scientific American, June 2019, 18-25. This is an edited summary.
[7] Ibid, 20.
[8] Ibid, 23-24.
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