In an ice cradle the first animals could have arisen during the global glaciations

(ORDO NEWS) — The time of the birth of the most ancient multicellular creatures is the subject of heated debate. In a new article, paleontologists discuss a bold hypothesis that helps explain some of the discrepancies in this area.

According to her, the first multicellular ecosystems (including the so-called inverted benthos) arose much earlier than is commonly believed – in the harsh conditions of the late Proterozoic, when the Earth was covered with ice at least twice from the poles to the equator.

The appearance of the first animals (in Latin – Metazoa ) and other multicellular creatures is one of the key moments in the evolution of the biosphere.

At the same time, the transition to multicellularity happened quite late: if the first signs of life are more than 3.4 billion years old, then the earliest animals (or something strongly resembling them) arose only 600-800 million years ago – perhaps a little earlier.

It turns out that for most of its history, the Earth’s biosphere was represented exclusively by prokaryotes and unicellular eukaryotes.

However, then this “planet of microbes” was transformed beyond recognition – and in the future, its evolutionary development was determined by multicellular creatures: animals, various mushroom-like organisms, algae and plants.

After a couple of hundred million years, already in the early Paleozoic, they began to explore the land and created a huge variety of terrestrial and soil ecosystems.

What was the reason for the transition of living things to a multicellular organization? This is a difficult question that biologists and paleontologists are still scratching their heads over. According to the geological record, the first multicellular animals appeared 572-602 million years ago.

However, if we turn to bioinformatics, which reconstructs evolution based on the DNA sequences of modern animals (the “molecular clock” method), then multicellular life arose another 250-286 million years earlier.

There are discrepancies in terms, and considerable ones. Additional questions are connected with the finds of individual sponges and some other organisms older than 800 million years – however, raising doubts.

To deal with this uncertainty, as well as the problem of the early evolution of multicellular life in general, researchers from the UK looked at the role of environmental factors in those long-standing events.

More precisely, they took into account exceptionally strong, even global glaciations – after all, it was then, at the very end of the Proterozoic, that the Earth twice turned into a “snowball planet” (snowball Earth).

In an ice cradle the first animals could have arisen during the global glaciations 2
Reconstruction of Late Proterozoic marine ecosystems and their inhabitants

As is known, the earliest described in detail and at the same time quite diverse forms of multicellular life belong to the Ediacaran period (541-635 million years ago), the last in the Proterozoic era.

They were strange creatures of the highest degree, like woven mats, swirling three-pointed stars and fractals of living flesh – they were called vendobionts. Unlike the later inhabitants of the Earth, the Ediacarans were preserved exclusively in the form of imprints of soft tissues – they did not have skeletons.

The origin of the inhabitants of the Ediacaran seas is no less mysterious than they themselves. Approximately 600 million years ago, they were already present in different parts of the planet – from the White Sea to the coast of Namibia and the Canadian Newfoundland (which, however, were not at all where we are used to seeing them on a modern map).

The “classic” inhabitants of the Ediacaran seas were preceded by a not too similar, but still quite large Lantian biota, known only from deposits in China.

When did the monotonous world of bacteria and other microbes become inhabited by multicellular organisms? Maybe in that period of the Neoproterozoic, which preceded the Ediacaran, in the Cryogenian?

But cryogeny (635-720 million years ago) was a difficult time for all living things: the planet spent almost the entire period in the form of a “Snowball Earth”, under the cover of global glaciation.

Thick, on average one kilometer thick ice has grown from the poles so much that they closed at the equator. Only in places this ice could contain polynyas, or at least be thinner.

In an ice cradle the first animals could have arisen during the global glaciations 3
Bradgatia sp.

Cryogeny began with the grandiose Sturtian glaciation, which marked the beginning of this period 720 million years ago and lasted 50-60.

This period was completed by the next, Marinoan glaciation (marinoan glaciation) lasting 15 million years. Thus, up to 75 of the 85 million years of the total duration of this period, the Earth spent in a “deeply frozen” state.

Against the backdrop of the monstrous frosts of cryogeny, the Gaskier glaciation (gaskier glaciation) that followed the Ediacaran seems to be moderate – “only” 340 thousand years of continuous ice, reaching the subtropics. This glaciation is also known as “slushy Earth” (slashball Earth).

The Frozen Earth of the late Proterozoic, to put it mildly, does not seem like a suitable place for new life forms to flourish. However, this is not entirely true – and the authors of a new article in the journal Global Change Biology describe this in detail.

Discussing the dramatic events on Earth in cryogeny, they note that it could be the site of an unprecedented evolutionary leap – the emergence of multicellularity. And its inhospitable conditions could contribute to this.

As a kind of model, the authors turned to the example of modern polar ecosystems, namely, Antarctica (which spent more than 15 million years under a two-kilometer layer of ice) and the surrounding shelf and floating ice.

These extreme habitats and their inhabitants are reminiscent of the planet during the cryogenian and, even more so, the Gaskier glaciation in the Ediacaran period.

Under a thick layer of ice (over 20 meters), photosynthesis is impossible, and oxygen access is very difficult. However, microbial communities, associated immobile filter feeders and active predators, now thrive in Antarctica under such conditions.

Such ecosystems can form even at a distance of hundreds of kilometers from the nearest polynya, moreover, in the dark.

The local inhabitants feed either on organic remains (detritus), which water circulation brings from afar, or through chemosynthesis – the use of the energy of chemical transformations by autotrophic prokaryotes. The same could have happened in the Late Proterozoic.

The described hypothesis to some extent explains the suspiciously early dating of the “molecular clock”, according to which at that time multicellular organisms already existed and even actively formed new systematic groups.

The article also made an original suggestion: some Ediacaran ecosystems could have formed … under water near the lower surface of the ice . Specific filter-feeding organisms like charnias ( Charnia sp. ), other rangeomorphs and other amazing vendobionts could be attached to it .

Such a community can be conditionally called “inverted benthic “, in which the role of the “bottom” is played by ice above.

Analogues of such ecosystems have been found in modern Antarctica: anemones Edwardsiella andrillae and some isopods are attached to the ice surface from below. Bacterial communities have also been found there and swimming predators feed on them.

In addition, the bottom of the Ediacaran seas at great depths could also be inhabited – thanks to the influx of oxygen from the saline waters descending from the surface. Sinking organic matter could serve as food for the local benthos.

It follows from the article that the deep parts of the ocean were a safer place for the very first animals , including because of the lower risk of being crushed by ice. And it was from there that the resettlement of animals to other habitats could begin.

The researchers also suggest that the Late Proterozoic ocean was actively mixed even at great depths.

Of course, transferring what is happening in modern Antarctica to the ancient frozen seas is possible only with reservations.

However, it should be remembered that the basic ecological laws – those that describe the relationship of living beings with each other and with the environment – are universal in nature and are valid in various parts of the Earth and in various periods of geological history.

In addition, this “Antarctic model” may be suitable for describing what is happening outside of our planet. As you know, a number of satellites of gas giants have endless oceans under a thick ice cover.

These are Europa, Callisto and Ganymede (moons of Jupiter) and Enceladus , orbiting Saturn. It is possible that similar conditions may arise in distant space oceans, suitable for the development of primitive life.

It is known that some of these moons, reminiscent of the ancient “Snowball Earth”, are characterized by both volcanism and geysers, and the presence of nutrients – important ingredients of the “living cell recipe”.

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