LIFE, MASS EXTINCTIONS, THE ANTHROPOCENE. 3rd part (multicellular organisms)

Post n. 42 English

1st Part etichetta Zga, 2nd Part etichetta 

The eukaryotes, appeared about 1.5 billion years ago, after about 750 million years have given rise to multicellular organisms from which, for subsequent evolutions, appeared plants and animals. Of these multicellular organisms, intermediate between the eukaryotic cell and plants and animals, we have no credible fossil record. There are single publications in which the author claims to have found fossils of multicellular dating back 800 million years, or 1.2 billion years ago and some go as far as 2.1 billion years ago. The data provided by these authors are scarce and of dubious interpretation. The scientific community therefore believes that the multicellulars have appeared about 750 million years ago without leaving any trace, at least for the moment, of fossil record.

It is certain however that the appearance of multicellular organisms and therefore of plants and animals was the fourth innovative turning point, which upset and, for better or for worse, continues to upset the planet.

But how did multicellular organisms originate?

It is likely that single cells of eukaryotes grouped together in a colony in cooperation for survival began to divide roles by freeing themselves from multiple duties and becoming a single organism. Why this evolutionary leap, which resulted in the fourth innovative breakthrough in the history of life and forever changed the evolutionary landscape of the earth, we do not know with certainty. However, we do record that right around that time the concentration of oxygen in the atmosphere went from 1/100th of 1.5 billion years ago to 1/10th of what it is today, determining at the same time an increase in the oxygen content of the oceans. It is likely that the increase in oxygen availability provided individual cells with the additional energy needed to enhance cellular systems and thus the evolutionary drive to support body structures.

In the last century, three great discoveries of ancient fossils were made: Animal fossils. Since these fossils have engaged geologists, palaeontologists, and evolutionists in long and often bitter confrontations for over a century, we will attempt a summary of the data in this article.

The first discovery of animal fossils dates back to 1909 by palaeontologist Charles D. Walcott, the Burgess fossils; the second dates back to 1947 by R.C. Sprigg, the Ediacara fauna and the third in Russia known for a long time and around 1960 called Tommotiana.  Since the ozone shield had not fully formed the landmasses were inhospitable places, these organisms were therefore exclusively aquatic.

However, in order of geological time the second is the oldest.

In an Australian area called Ediacara, Sprigg discovered a soft-bodied fauna that took the name of Ediacara fauna and dated 575 million years ago. This fauna even if it takes the name of Ediacara,

was actually spread all over the planet and represents the oldest evidence of multicellular organisms similar to soft-bodied animals that is they did not have internal skeletons or external armours. They were flat and motionless organisms which developed in length, reaching even one meter, in width in the shape of leaves with height and width even of one meter and radial with specimens which reached even 30 cm. Probably, as some researchers believe, the flat shape allowed for greater contact with water and the ability to absorb more nutrients and oxygen for their vital functions.

As reported by Stephen J. Gould (*) in "The Wonderful Life the Burgess Fossils and the Nature of History" 1990, these fossils were thoroughly described by palaeontologist Martin Glaessner who interpreted them as primitive representatives of modern animal groups. Nevertheless, palaeontologist Dolf Seilacher proposed a completely different interpretation. And Gould reports, "Seilacher argues that most Ediacara animals can be taxonomically joined together as variations on a single anatomical plan: a flattened form in sections that are woven or sewn together, perhaps constituting a hydraulic skeleton much like a pneumatic mattress. Since this design does not correspond to any modern plan, Seilacher concludes that the Ediacara organisms represent an entirely separate experiment in multicellular life: an experiment that ultimately ended in a previously unrecognized late Precambrian extinction, since none of the Ediacara elements survived into the Cambrian." Today many scientists still share this hypothesis: Ediacara was a failed experiment for life.

But how long did the Ediacara fauna live?

Approximately 40 million years. With due caution, perhaps they have not left descendants but if they have lived for 40 million years the experiment is not that much failed.

The same fate befell the Tommotiana fauna in Russia also widespread in various areas of the planet. This fauna in addition to multicellular organisms with soft body, contained organisms with hard parts called "small fauna with hard parts". Initially these fossils were interpreted as precursors of modern animals but perhaps only some are.

With the Burgess fossils, we enter in full into the animal kingdom. The fossils extracted by Walcott in 1909 from the Burgess quarry are about 8000 and have been dated 540 million years ago, that is in that geological period called Cambrian that begins about 542 million years ago. In the decade following the discovery, Walcott studied and classified all the fossils as ancestral forms of modern animals. It also seems that the typical body organizations of animals (phylum) have all appeared in that period and for this reason it is called "Cambrian explosion". In 1971 Harry Wittington, Simon Conway Morris and Derek Briggs after a study on Burgess Fossils proposed a re-examination of Walcott's conclusions. In particular, according to these researchers Burgess fossils contain both animals with body plans attributable to modern animals, but contain animals (at least 15 phyla) that do not belong to group of animals known to us and therefore have subsequently become extinct.

This review gave to S.J. Gould the cue to highlight some limitations of Darwinian theory. In (work cited) he points out that the Darwinian theory of a slow and gradual change towards complexity was an idyllic vision and that things are actually quite different. Gould starts at the origin of life 3.5 billion years ago and highlights how for 2 billion years life remained at the prokaryotic level without any evolution towards complexity. The appearance of the eukaryotic cell 1.5 billion years ago did not lead to a slow evolution towards complexity but it took at least 800 million years before the appearance of multicellular organisms. And Gould points out that even Burgess' review of fossils shows that slow and gradual evolution did not occur if some evolutionary lines became extinct.

Now, with respect to Burgess' fossils, the problem Gould poses is this: why did some animals go extinct while others progressed to complexity? 

He reports the conclusions of Conway Morris shared by Briggs and Whittington: "It may be that diversification is simply a reflection of the availability of a nearly empty ecospace with low levels of competition, which allowed the evolution of a wide variety of body plans, only some of which survived in the increasingly competitive environments over geologic time. (1985, p- S70)." That is, the Darwinian view of survival of the fittest. But according to Gould in order to establish why these animals were successful, one must be able to identify in them some anatomical excellence of their own, some structural advantage. He then reports what Conway Morris wrote and shared in principle by Whittington «A hypothetical observer in the Cambrian would presumably have had no means of predicting which of the metazoans were destined for phylogenetic success as established body plans and which were doomed to extinction" (1985, p. 572)». Gould then draws his conclusions: if nothing indicates to us that one was better than the other the choice of body plans that led to modern animals must have been a random choice. Conclusion that then culminated in the now famous sentence: «I believe that the reconstructed Burgess fauna, interpreted in light of the theme of the repetition of the film of life, offers strong support for the following different conception of life: any repetition of the film would lead evolution down a radically different path from the one actually taken. [...] If we repeat the film of life a million times, starting with Burgess, I doubt that anything like Homo sapiens will ever develop again».

As we have already mentioned at the time Gould expressed his ideas the Ediacara fauna was unanimously considered a failed independent experiment, and for Gould the Tommotian fauna was also perhaps a failed experiment. All of this reinforced by Burgess' review of the fossils contributed to the idea that contingency played a decisive role in the history of life.

After the initial acclaim of many researchers and philosophers of science for Gould's hypotheses, a few years of further research changed some of the frames of the movie of life envisioned by Gould.

To distinguish whether a pluricellular fossil belongs to a phylum of animals, it is necessary that the fossil presents first of all a bilateral symmetry typical of animals and if possible traces of appendages, mouth and intestine. Unfortunately, the latter are very difficult to identify because in the long term the fossils have undergone drastic processes of alteration. A few years after the publication of Gould's essay, Jeffrey S. Levinton clarifies it well in "The big bang of animal evolution" Le Scienze 1993: «To decide instead that a fossil belongs to a known group it is necessary to find the diagnostic characters that demonstrate the correlation. When fragmentary fossils of an unknown organism are discovered, often these characters are absent, [...] and therefore potentially subject to reinterpretation».

In the same article Jeffrey S. Levington reports further studies on two fossils, Wiwaxia, an organism with spines about two and a half centimetres long, and Hallucigenia, which

Wikipedia

Conway Morris had assigned to completely new and extinct evolutionary lines, but which in fact were found to belong to phylum of current animals. And Levington concludes: « [...] since several fossils are fragmentary and therefore potentially subject to reinterpretation, I believe that Gould (*) and with him other palaeontologists have exaggerated about the diversity of life forms existing in the Cambrian».

In 2005 in "The evolution of the first animals”, The Sciences David J. Bottjer reports of findings of his research group in a region of China called Doushantuo in rocks dating back to 580 million years ago, in full ediacarian era. Bottjer says they found 10 specimens of an oval-shaped soft-bodied organism, which they named Vernanimalcula that has bilateral symmetry and a gut with associated mouth and anus.

Fossils found in Newfoundland, a Canadian island in the Atlantic Ocean, dated to 571million years ago, have been recognized by most scientists as animal fossils.

To baipass fossil alteration processes, Ilya Bobrovskiy relied on biomarkers. As reported in "The oldest animal is 558 million years old" Science 2018, in two localities in Russia called Lyamtsa and Zimnie Gory Ilya Bobrovskiy and colleagues identified two fossils called Dickinsonia and Andiva that appeared to have undergone less drastic alteration processes. By analysing the fossil samples, the researchers identified traces of lipids and sterols typical of animals. The fossils are found in a rock matrix dated 558 million years ago, more than fifteen million years before the Cambrian and therefore also in the middle of the Ediacarian era.

Paleontologist Rachel A. Wood in "The Big Bang of Life" Science 2019 reports on a fossil of about 7 cm called Cloudina, which appears not to belong to any phylum of modern animals discovered Namibia and dated to 550million years ago in the Ediacaran epoch. Cloudina was later found in Cambrian times in both China and Siberia.

In November 2020, Le Scienze on line reports an article by Traci Watson/Nature. The author after highlighting the work of Bobrovskiy reports the high-resolution three-dimensional resolutions of Mary Droser, University of California, of an organism, contemporary of Dickinsonia that is 558 Million years ago, called Ikaria Warootia the 

size of a grain of rice that has both bilateral symmetry and digestive tract and able to move and dig burrows.

To be correct we must add what Rachel Wood writes in (cited article): «Usually to date the rocks of that period is measured the ratio uranium-lead in zircons found in the nearby layers of ash derived from ancient volcanic eruptions. It is one of the few methods that can return the absolute radiometric age of a rock. Unfortunately, however, in many of the best known successions these very important ash layers are not found, so it is not possible to accurately correlate the evolutionary changes that occurred in different parts of the world, which is essential to create a solid time frame for the succession of events we want to describe».

Thus, to get the date of a site we often refer to lower or upper layer dates. Dates obtained in this way present approximations of about 2 million years.

However, the discoveries of the Ediacaran period in China, Russia, and Canada have led several scientists to question whether the Ediacaran period was a failed experiment.

If, as evidenced by research, animal fossils in addition to the Cambrian are found in the Ediacaran and Ediacaran organisms are found among the animals of the Cambrian, there was no clear separation between the Ediacaran, Thommotian, and Cambrian worlds. These worlds interpenetrate and there was not a Cambrian explosion but an explosion of animal life. Probably as Conway Morris wrote, the empty ecospace allowed for the evolution of a wide variety of body plans, only some of which survived in the increasingly competitive environments over geologic time.  Competition favoured the most adapted organisms.

But why were the most adapted organisms initially the ediacarans?

About 600 million years ago, the earth began to move out of its snowball Earth condition. As recent research points out, Pannotia, the supercontinent that united all landmasses at that time, began to break up giving rise to shallow seabeds rich in inorganic nutrients. Oxygen in that era went from 1/100 to 1/10 of what it is today. It is therefore possible that at that time the ediacarans, immobile and with their large surfaces, were the most suitable to absorb nutrients and especially oxygen for their metabolism, while the animals with their apparatus struggled for the same functions. As Rachel A. Wood reports (article cited above) research to estimate the oxygen content of the seas of that era found that: "The shallow seafloors that were habitable for early animals, therefore, were even more limited than researchers expected; they were veritable oases of oxygen-rich water." But from 600 million years ago to today, oxygen has passed, albeit with ups and downs to the current content.

So it is possible that after a few tens of millions of years, slight increase in oxygen made the animals more adapted. If we add that the animals had the ability to move and that at 

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that time predators such as Anamolocaris also appeared, you can imagine the end of the docile and static Ediacarian fauna.

After 100 million years of their appearance, in a period that geology calls Ordovician, animals suffered their first mass extinction that led to the disappearance of more than sixty of the animal species. What were the causes? It is not known with certainty perhaps a glaciation that lowered the level of the seas that caused the extinction of many species. Glacial deposits of this period were found in various parts of the earth. According to a study conducted by researchers from the Universities of Exeter, Oxford, East Anglia and the John Innes Centre published in "Nature Geoscience" and reported by Le Scienze 2012, it was the appearance and colonization of dry land by the first plants that, around 450 million years ago, with a massive absorption of CO2 altered the carbon cycle, causing a series of ice ages.

The appearance of plants caused, perhaps, a mass extinction but definitely changed the face of the planet, and thanks to them we are here today.

And here begins another story 

                                                                                Giovanni Occhipinti                                 

 

(*)

Stephen J. Gould professor of zoology and geology at Harvard University was one of the greatest and most influential evolutionists of the last century. He elaborated, together with Niles Eldredge in the seventies of last century, the theory of "punctuated equilibrium". He argued that evolution does not always follow a slow and gradual progress towards complexity as Darwin thought, but alternates long periods of stagnation of millions of years, in which no changes are observed, to rapid events in which new species originate, i.e. punctuated processes. The theory of punctuated equilibrium focused the debate of evolutionists until the end of the last century. This was also a period of fierce debates, where often instead of entering into the merits of the theory, it was involved in ideological arguments completely unrelated to science. With Elisabeth S. Vrba published "Exaptation”. The bricolage of evolution" to explain how organisms readapt existing structures to perform new functions. He wrote numerous popular essays and was considered one of the most influential science popularizes of his time.

Next article: Plants and animals, the conquest of the continents. It will be released mid-April