Our Common Ancestors
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Our Common Ancestors

A Tale of the Cryogenian

Published in The EarthSphere Blog. Feather Image: View from Below the Cryogenian Ice (by ArcheanArt)

Prologue

Our last article in the Forgotten Origins series discussed the Cryogenian Period from 720 to 635 million years ago. During this interval, our common animal ancestors first appeared. Was the Cryogenian a dividing line between single-cell life and complex multicellular organisms?

The Simplest of Creatures

The farther back in time we go, the greater the difficulty in identifying fossilized animal remains because there were no hard parts like bones or shells to preserve in the earliest fossil record. However, many scientists believe sponge-like creatures were the first animals on the zoological evolutionary tree. Yes, somewhere in the dim annals of history, your earliest ancestors were brainless sponge-like creatures hanging around together, thinking about absolutely nothing.

The evidence for this claim is based on molecular phylogeny, a technique using DNA molecules to determine the evolutionary history of genes and species. Extrapolations from phylogenomic analyses point to the first ancestral sponges appearing during the Cryogenian. 

These ancient creatures didn’t have any true hard parts, but they probably had tough protein fibers called spongin, which formed a three-dimensional network. Spongin is preserved in some circumstances when it calcifies during the decay process after a sponge is dead and buried beneath sediments. This calcification leaves a fossil signature in the rock once the sediments solidify. Fossil signatures for sponges are well known in Cambrian limestones younger than 540 million years.

In 2015, researchers presented evidence of a well-preserved ancestral sponge fossil from a 600 million-year-old rock formation in South China. The fossil was exceptionally well preserved, allowing for the observation of cell types and cell structures in the remains. The fossilized animal was only about 3 millimeters in size, but it was unmistakable as an ancient sponge.

The Cryogenian Divide

The Cryogenian may form a dividing line in the evolution of our biosphere. Before then, single-cell life was the rule, but after the planet thawed, we entered the Ediacaran Period, where complex, multi-celled organisms appeared and spread across the oceans. Something happened in the cold, dark reaches below Earth’s frozen oceans.

The Nature article ‘Cryogenian magmatic activity and early life evolution’ (Jie Long, et al.) expresses this division well:

“Before the Cryogenian period, life was simple organisms, but afterward, evolutionary processes were characterized by a progressively accelerated stage, with life becoming more complex from 635 to 520 million years ago.”

But despite the speculation and theories about a Cryogenian divide, we don’t know if life’s step forward in the Cryogenian was simply coincidental or whether environmental conditions sparked a change. Maybe the groundwork was already in place for animal life when Sturtian glaciation kicked off. Geologist and paleobiologist Elizabeth Turner (Laurentian University) believes she has uncovered a fossilized spongin network in 890-million-year-old rocks from northern Canada. If she is correct, these fossils will be the earliest animal life yet discovered — animal life that preceded the Cryogenian.

But the Cryogenian does become a significant divide if evolution accelerated during the period due to changes in environmental conditions. Such changes imply the introduction of chemical components necessary for complex life to develop and thrive. We know that the presence or absence of trace elements and nutrients in today’s oceans controls both algae and plankton populations. The reasons behind the Cryogenian divide may relate to chemical changes in Earth’s ancient oceans.

Cryogenian Oceans

The Cryogenian period followed the breakup of the Rodinia supercontinent. Tectonic plates separated and drifted apart, allowing magma to upwell from Earth’s mantle, resulting in large-scale plate margin volcanism. A group of Cryogenian rocks in China named the Quinling Orogenic Belt records the geological history of a large magmatic-volcanic event.

Analysis of these volcanic rocks shows they are enriched in a variety of trace elements when compared to average felsic volcanics. Researchers studying the Quinling rocks proposed that increased access to these trace elements in the post-Rodinia oceans provided nutrients essential to the evolution of complex life.

It’s an interesting proposition. If the planet’s surface was encased in ice, then life might retreat to areas deep in the ocean where magmatic or hydrothermal vents provided a continuous supply of heat and nutrients. Perhaps a combination of environmental stress and ample supplies of trace nutrients forced dramatic evolutionary changes as life struggled to cope with an ice-bound environment. Before the Cryogenian photosynthetic algae dominated the biosphere, but their access to sunlight was dramatically curtailed as ice sheets spread from the poles to the equator. It was an environmental crisis and possibly a period of mass extinction.

One of the repeated patterns we find throughout Earth’s history is that rapid environmental changes stress life and cause massive species extinction. Life doesn’t disappear, but it is severely choked back. Life returns once the crisis abates, but not in its prior form. After each mass extinction, we see the rise of new life forms and species. Perhaps the Cryogenian followed this pattern, and a world dominated by only algae was replaced by ocean floors covered with the new and wonderful organisms of the Ediacaran.

(Next — The Ediacaran)

ArcheanWeb
ArcheanArt

Sources:

An 890-Million-Year-Old Fossil Sparks a Radical Theory About Life (by Elizabeth C. Turner; Inverse)

A new fossil discovery may add hundreds of millions of years to the evolutionary history of animals (Elizabeth C. Turner; The conversation)

Cryogenian magmatic activity and early life evolution (by Jie Long, Shixi Zhang & Kunli Luo ; Scientific Reports)

Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian (by Zongjun Yin, Maoyan Zhu, Eric H. Davidson, David J. Bottjer, Fangchen Zhao, and Paul Tafforeau; PNAS) 

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.