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The Rise of Animal Life

It Started With a Sponge

A classic view on the origins of animals holds they entered onto the stage of life about 540 million years ago (mya) in an event called the Cambrian Explosion. After contemplating the move for the first four billion years of Earth’s existence, enough evolutionary momentum developed for life to take a bold step forward from single-celled algae and bacteria to complex multi-cellular life. The assignment of 540 million years as the starting point was based on the appearance of animal fossils in the rock record. It turns out that 540 million years more likely represents the point when animals developed hard parts like shells and other skeletal supports. Some scientists now believe the first real animals may have appeared some 890 million years ago in the form of sponges.

Complex animal life on Earth has a basic chemical limitation; it requires free oxygen. While we take oxygen for granted, it’s important to remember free oxygen wasn’t present for much of Earth’s history. Its arrival as a viable component of the atmosphere is known as the Great Oxygenation Event, whose age is placed at 2.33 billion years ago by a group of MIT scientists. But even then, oxygen levels were low, and the deep oceans probably remained anoxic. A separate piece of research from Berkeley posits that complete oxygenation of the atmosphere didn’t occur until between 540 and 400 million years ago. This timeline leaves us with a stretch of about 1.7 billion years when oxygen was present but in small and probably varying quantities.

Despite the lower percentage of free oxygen available in the oceans and atmosphere before the Cambrian Explosion, we know life was on the move during the Ediacaran Period between 653 mya and 541 mya.

The Ediacaran

When Darwin wrote his masterpiece “On the Origin of Species” in 1859, no fossils older than the Cambrian were known. It was 1957 before definitive evidence of complex Precambrian lifeforms was confirmed. Another 30 years passed before Adolf Seilacher proposed that Ediacaran biota were not truly animals but a failed evolutionary branch he called ‘Vendobionta.’

The Ediacaran Period followed the Late Cryogenian when Earth froze over from pole to equator (Snowball Earth). As the planet thawed out, soft-bodied life took over the shallow oceans. The Ediacaran was a critical period when life mapped out new pathways like guts, legs, and complex behaviors such as burrowing.

Today scientists are more inclined to see the Ediacarans as an eclectic group of organisms, representing multiple evolutionary pathways — mother nature was rolling the dice to see what might turn up. Included within this grab bag of lifeforms were the early precursors of animal life as we know it today. But were they the first animals?

The Simplest of Creatures

The farther back we go in time, the greater the difficulty in identifying fossilized animal remains because there were no hard parts to preserve in the earliest fossil record. However, many scientists believe sponge-like creatures were the first animals on the zoological evolutionary tree. Their evidence for this claim is based on molecular phylogeny, a technique using DNA molecules to determine the evolutionary history of genes and hence species.

These early creatures didn’t have any true hard parts, but they probably had tough protein fibers called spongin, which formed a three-dimensional network. Research has previously shown that spongin is preserved in some circumstances when it calcifies during decay after a sponge is dead and buried beneath sediments. Eventually, this process leaves a fossil signature in the rock when the sediments solidify. This fossil signature for sponges is well known in limestones younger than 540 million years.

Geologist and paleobiologist Elizabeth Turner (Laurentian University) believes she has uncovered this same fossilized spongin network in 890-million-year-old rocks from northern Canada. If she is correct, these fossils would be the earliest animal life yet discovered. Yes, somewhere in the dim annals of history, your earliest ancestors were brainless sponge-like creatures hanging around together, thinking about absolutely nothing. These characteristics may make them virtually indistinguishable from some of today’s congressional committees.

Related Articles:

A billion years of missing oxygen (by WM House; ArcheanWeb) 

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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) 

Mantle data imply a decline of oxidizable volcanic gases could have triggered the Great Oxidation (By Shintaro Kadoya, David C. Catling, Robert W. Nicklas, Igor S. Puchtel & Ariel D. Anbar; Nature

When and where did Earth get its oxygen? (By Eleanor Imster; EarthSky) https://earthsky.org/earth/when-where-earths-o2-oxygen-rise-began/

Which came first: complex life or high atmospheric oxygen? (by Robert Sanders; Berkeley News) 

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.