Freeze and evolve

Updating The Chilling Stars and Magic Universe

When cosmic rays freeze the world, you’d better evolve

2100 million year-old multicellular fossil found in Gabon. Image: Kaksonen CNRS

Transforming the story of life on the Earth is a report in Nature today about multicellular creatures more than 2 billion years old, at a time when single-celled bacteria supposedly reigned supreme. Fossils you can pick up with your fingers, found in Gabon, West Africa, are far, far older than the multicellular animals that become detectable about 600 million years ago (Ediacaran period) and conspicuous 542 million years in the “Cambrian explosion”. The age is fixed with remarkable precision at 2070 to 2130 million years.

Exterior and interior of a fossil imaged by micro-tomography. Image: El Albani & Mazurier, CNRS

A team of 21 experts from France, Sweden, Denmark, Canada, Germany and Belgium make the report. The lead author is Abderrazak El Albani, at the University of Poitiers, France. He tells Agence France Press that “More than 250 specimens have been found so far. They have different body shapes, and vary in size from one to 12 centimetres.”

What excites me about the discovery is that here was a far-reaching evolutionary response to the rise of oxygen in the Earth’s atmosphere beginning more than 2000 million years ago. It occurred in the aftermath of a planet-wide freeze for which there is a cosmic explanation.

Chapter 6 in The Chilling Stars includes the story of “Snowball Earth” events. Here are some extracts.

In 1986, George Williams and Brian Embleton in Australia used the magnetism in grains of iron oxide dropped from ancient ice to show that they were released within a few degrees of the Equator. A few years later, Joseph Kirschvink of the California Institute of Technology confirmed this result in magnetism associated with other rock formations in Australia produced by ice action, and well dated as 700 million years old. He called it ‘bullet-proof evidence’.

“It now seems clear that these extensive, sea-level deposits … were formed by widespread continental glaciers which were within a few degrees of the equator. The data are difficult to interpret in any fashion other than that of a widespread, equatorial glaciation.”

Kirschvink invented the name Snowball Earth for that dire climatic state. You have to visualise ice sheets, glaciers and frozen seas even at the Equator itself. The degree of ocean freezing is still debated. Some investigators imagine vistas of ice a kilometre thick or more, others prefer a ‘slushball’picture with drifting sea ice and icebergs. Either way the impact on life was severe.

Evidence from all the world’s continents unpacks into about three separate snowball episodes in the interval 750 to 580 million years ago. Worms that survived by scavenging the sea-bed detritus evolved the body-plans that made possible the explosion of animal life mentioned in the previous chapter, when the world became reliably warmer again in the Cambrian Period that started 542 million years ago.

Those cold Neo-Proterozoic times, as geologists call them, were not the only occasion of such radical events involving ice and evolution. By the end of the 20th century, geologists had amassed evidence from South Africa, Canada and Finland that confirmed two Snowball Earth episodes between 2,400 and 2,200 million years ago, in Palaeo-Proterozoic times. Our planet was then only half its present age.

Remarkable souvenirs from around that time include the world’s largest deposits of iron and manganese ore, produced by the action of oxygen on those metals dissolved in sea water. The whole planet rusted. Many ancient lineages of bacteria were wiped out on Snowball Earth, but novel microbes called eukaryotes survived the massacre. These were single-celled fungi, algae and animal-like grazers distinguished by their use of cell nuclei to encapsulate their genes. By 1,800 million years ago some eukaryotes had taken in oxygen-handling bacteria to serve as power stations, in cells of the modern kind now found in every plant and animal. The descendants of those bacterial lodgers are present in your body as mitochondria. Their antiquity is manifest. They originated before sex was invented, and you inherited them only from your mother.

The big geochemical and biological events associated with the climatic extremes of Snowball Earth prompt debates about cause and effect. One scenario for the Palaeo-Proterozoic freezes was that over-zealous production of oxygen by bacteria caused the rusting episode and somehow triggered the icing of the planet by altering the composition of the atmosphere.

But the main challenge for anyone wanting to account for the snowball events is to say why they occurred at particular times in the long history of the Earth, in two relatively brief windows of time around 2,300 million and 700 million years ago. A complete solution to the conundrum should also say why, between those occurrences, our planet was completely free from ice for a billion years.

The chilling stars provide the only explanation for Snowball Earth that specifies these timings. After Nir Shaviv in Jerusalem had accounted for the hothouse–icehouse alternations in climate in the past 500,000 years, by visits to the spiral arms of the Milky Way, his next step was to link the snowball events to episodes of star-making in the Galaxy. These boosted the cosmic rays to such exceptional levels that the Earth became cloudy and sunless enough to freeze over.

[An astronomical passage explains starburst events of exceptional star-making, and their detection in our own Milky Way Galaxy.]

Meanwhile, what stands out is the correspondence in time between the early Snowball Earth episodes of about 2,300 million years ago and Rocha-Pinto’s starburst in the period 2,400 to 2,000 million years ago. There are reasons for suspecting that the two events were connected, by the unusually high cosmic rays to which the Earth was subjected. But if this was more than a chance coincidence, then the ice-free interval that followed should be associated with a scarcity of stars born at that time. For Shaviv this was a key point in his argument.

“The long period of 1 to 2 billion years before present, during which no glaciations are known to have occurred, coincides with a significant paucity in the past star formation rate.”

And the later Snowball Earth episodes starting around 750 million years ago should also be linked to another stellar baby boom. Rocha-Pinto’s census of Hipparcos stars does indeed show star-birth much reduced between 2,000 and 1,000 million years ago. But the rate of star formation that follows the lull, in this census, is not very impressive. More persuasive are the results of another survey announced in 2004 by Raúl de la Fuente Marcos of Suffolk University, Madrid, and Carlos de la Fuente Marcos of Universidad Complutense de Madrid. They used data on groups of stars called open clusters, as catalogued by astronomers over many years, to infer among their other results a starburst around 750 million years ago. The Fuente Marcos pair noted its timeliness for Shaviv’s story.

“The Snowball Earth scenario appears to be connected with the strongest episode of enhanced star formation recorded in the solar neighbourhood during the last 2,000 million years.”

Here is extraordinary support for the idea that cosmic rays have controlled the climate throughout the Earth’s history. When a hypothesis is false, new experiments and observations will tend to quarrel with it, but with a good theory the reverse is true. It looks better and better as the facts become more exactly known.

Today’s update adds here:

An uncanny twist to the story came in 2010, with the discovery of the earliest known multicellular creatures that you could pick up with your fingers. They incorporated two huge evolutionary leaps: modern cells with nuclei (eukaryotes) and the organization of those cells into functional bodies. And they are dated at 2,100 million years ago, in the aftermath of the second Palaeo-Proterozoic snowball 2,200 million years ago.

What’s uncanny is the correspondence to the emergence of the oldest previously known animals, in the Ediacaran period about 600 million years ago, preceding the “Cambrian explosion” of animal life 542 million years ago. In both cases multicellular forms appeared in the wake of Snowball Earth events and a rise in oxygen in the atmosphere. If you’re a smart critter and cosmic rays freeze the world, it seems that you’d better evolve quite radically.

As for Magic Universe, the most relevant passage is the opening of the story called “Tree of life: promiscuous bacteria and the course of evolution.”

A stagnant pool was a treat for Lynn Margulis when, as a young biologist at Boston University, she liked to descant on the little green bugs that can so quickly challenge human notions about how a nice pond should look. Her favourites included the blue-greens, often called algae but in fact bacteria, which have played an outstanding role in steering the course of life on the Earth.

Margulis became the liveliest and most stubborn advocate of the idea that we are descended from bacteria-like creatures that clubbed together in the distant past. Others had toyed with this proposition, but she pushed it hard. In 1970 she published a book, Origin of Eukaryotic Cells, and she followed it in 1981 with Symbiosis in Cell Evolution. These are now seen as landmarks in 20th-Century biology, and the keywords in their titles, eukaryotic and symbiosis, go to the core of the matter.

You are the owner of eukaryotic cells. Each of the billions of microscopic units of which you’re built safeguards your genes of heredity within a nucleus, or karyon in Greek. So, in the grandest division of living things, into just two kinds, you belong to the eukarya. That groups you with other animals, with plants, with fungi, and with single-celled creatures called protoctists, represented by 250,000 species alive today and often ambiguous in nature.

In the other great bloc of living things, the prokarya are all single-celled, and the genes just slop about within them. The earliest forms of life on the planet were all of that relatively simple kind, meaning bacteria and similar single-celled organisms called archaea. They ruled the world alone for half its history, until the eukarya appeared.

Symbiosis means living together. The proposition for which Margulis first marshalled all the available evidence is that small bacteria took up residence inside larger ones — inside archaea, one would say now — and so formed the ancestors of the eukarya. Instead of just digesting the intruders, the larger cells tolerated them as lodgers because they brought benefits.

The outcome was the microscopic equivalent of mermaids or centaurs. ‘The human brain cells that conceived these creatures are themselves chimaeras,’ Margulis wrote with her son Dorion Sagan, ‘– no less fantastic mergers of several formerly independent kinds of prokaryotes that together co-evolved.’

Oval-shaped units inside your cells, called mitochondria, are power stations that use oxygen to generate chemical energy from nutrients. They look like bacteria, they carry sloppy genetic material of their own, and they reproduce like bacteria. The same is true of chloroplasts, small green entities found in the cells of the leaves of plants. They do the work of harvesting sunlight and using water and carbon dioxide to produce energy-rich molecules that sustain plant life and growth. [Irrelevant sub-heading omitted]

In the Margulis scenario, the ancestors of the mitochondria and chloroplasts were indeed bacteria that took up symbiotic residence inside other single-celled creatures. The mitochondrial forebears were bacteria that had learned to cope with oxygen. When that element first appeared unbound in the ancient sea it was deadly dangerous, like bleach poured into the bacterial-archaeal communities. So bacteria that were adapted to it could offer their hosts protection against oxygen and also the ability to exploit it in new ways of living.

Blue-greens, formally called cyanobacteria, were the ancestors of the chloroplasts. In their separate, bacterial existence, they had hit upon the most powerful way of using sunlight to grow by. It involved splitting water and releasing oxygen, and so the blue-greens were probably responsible for the oxygen crisis. But this smart photosynthesis also conferred on the hosts the capacity to generate their own food supplies.

Host cell plus mitochondria made the ancestors of fungi and of protoctists. The latter included some distinguished by their capacity for swimming about, which became the ancestors of the multi-celled animals. Host cell plus mitochondria plus chloroplasts made single-celled algae, and among these were the forebears of the multi-celled plants.

Aspects of the scenario are still debated. Especially uncertain is how all of these cells came to organize their cell nuclei, and how they perfected the eukaryotic kind of cell division used in multiplication, growth and sex. The origin of the capacity for movement in protozoa, and its possible survival in the swimming tails of sperm, is also controversial.

The broad brush-strokes of the symbiosis story are nevertheless accepted now. Not just as a matter of taste, but by verification. The kinship of identifiable bacteria with mitochondria and chloroplasts is confirmed by similarities in their molecules. Fossil traces of early eukaryotes are very skimpy until 1200 million years ago, but the molecular clues suggest an origin around 2 billion years ago, at a time when free oxygen was becoming a major challenge to life.

Today’s update inserts at this point.

The molecular tracing of evolutionary history was wonderfully vindicated in 2010 with the discovery in Gabon, West Africa, of the earliest known finger-sized creatures. As they were multicellular they had to be built of eukaryotic cells. And their fossils are 2100 million years old. Abderrazak El Albani at Poitiers, France, told a reporter, “More than 250 specimens have been found so far. They have different body shapes, and vary in size from one to twelve centimetres.” The creatures he described as “cookie-like” gave fossil-hunters a new puzzle. Why did the multi-celled eukaryotes remain inconspicuous for 1500 million years?

There should probably be a sentence or two added also to another story, “Cambrian explosion: easy come and easy go, among the early animals”, which refers to soft-bodied multicellular creatures appearing about 600 million years ago.

References

Abderrazak El Albani et al., Nature, 466, pp. 100-104, 2010

El Albani quote AFP http://www.google.com/hostednews/afp/article/ALeqM5j5tBsadjwEze-WH7rc2vhRgomesQ

H. Svensmark & N. Calder, The Chilling Stars, pp. 157-166, Icon Books, 2007

N. Calder, Magic Universe, pp. 681-683, Oxford UP, 2003

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