ArcheanWeb

The Ediacaran World

February 11, 2022

Biosphere, Earth Science, EarthSphere Blog, Environment, Forgotten Origins, Repost, Science

Animal life Thrives

Published in The EarthSphere Blog. Feature Image: Dickinsonia Imprint (by ArcheanArt)

Prologue

We last discussed the Cryogenian in the Forgotten Origins series. Earth went into a deep freeze, and somewhat mysteriously, animal life appeared during the cold spell. But the following Ediacaran Period is when animals started to thrive. I apologize to my consistent readers that the first section below largely draws from a previous 2020 article of mine, Before the Explosion.

Darwin Didn’t Know

When Darwin wrote his masterpiece “On the Origin of Species” in 1859, no fossils older than the Cambrian were known. It was the 1950’s 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 rather a failed evolutionary branch he called ‘Vendobionta.’

Today scientists are more inclined to view the Ediacaran as a world containing 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.

The Ediacaran Period occurred at the end of the Precambrian between 635 and 541 million years ago. Earth had thawed from the Cryogenian deep freeze, and as the planet warmed, soft-bodied life took over the shallow oceans. The Ediacaran was a critical period when life mapped out new pathways like guts, limbs, and complex behaviors such as burrowing.

Our delay recognizing 94 million years of evolution demonstrates that investigating Earth’s ancient past is a tenuous affair. Uncovering evidence of life in the geological record relies on a combination of unusual preservation circumstances and the random luck of fossil deposits being at Earth’s surface present day. One of the problems with preserving fossilized materials is the vicious competition for food within all ecosystems. Life takes dinner wherever it can be found and always devours it as completely as possible. All that is edible will be eaten as predators eat prey and scavengers scoop up what is left. Bacteria then feast on any remaining organic debris. The biosphere always tries to recycle life into new life. This fact makes the chance of dead animal remains being fossilized very small indeed.

Ediacaran Biota

The biota of this period is named for the Ediacara Hills of South Australia. Organisms of the Ediacaran Period represent the first big influx of metazoans into Earth’s biosphere (metazoans are animals made up of more than one type of cell). The earliest metazoans probably made their appearance in the Cryogenian. Still, the 94 million years following the big thaw were when evolution honed these creatures’ survival skills, allowing them to dominate their particular ecological niches.

Many of the Ediacaran animals lived in warm shallow seas. They enjoyed the benefits of sunlight and oxygen exchange between the atmosphere and the ocean. Atmospheric oxygen was a key component of the Ediacaran evolutionary saga. These early metazoans required free oxygen for cellular respiration, and oxygen may also have been a controlling factor in their preservation. Even though these organisms lacked any true hard parts, some of their cellular structures included collagen, making them tougher, more resilient, and more prone to fossilization and preservation.

Interestingly, animals can’t produce collagen when atmospheric oxygen levels are less than three percent of the modern levels. This fact leads to speculation that Earth passed this magical ‘three percent mark’ during the Ediacaran Period, allowing evolution to incorporate collagen proteins into the cellular mix. Prior to reaching this mark, all fossil evidence consisted of trace fossils like burrows or trails. We can read the geological record to see something was living there, but no evidence of the actual animal remains. After reaching the three percent threshold, imprints of the animals were preserved in the fossil record.

The Early Peeks at Animal Life

A 2018 article in Science focused on a 558 million-year-old organism called Dickinsonia, a broad, flat, leaf-shaped creature. We are not talking about a microscopic critter. These organisms grew up to four feet in length. Dickinsonia fossils have been known for over 70 years, but until recently, debate raged about whether it was a large amoeba, an animal, or even a plant. Simply looking at the visible remains didn’t make us any wiser.

But the science of studying Precambrian fossils has evolved to include more than simple visual analysis to understand what might be an animal versus something else. Geobiologists have entered the academic fray and developed techniques to isolate fat molecules from Ediacaran fossils. This line of investigation identifies chemical products from the breakdown of cholesterol — a product common in animal cell membranes.

Another group utilized 3D laser scanning technology to identify tiny, fossilized Ediacaran creatures (Ikaria wariootia) with left-right symmetry and identifiable frontends and backends. As much as we might not like to be reminded, a mouth, guts, and an anus are the basic defining characteristics of multicellular animal life, be it a worm or a human being.

Researchers studying Dickinsonia fossil remains isolated lipid biomarkers that were unambiguously indicative of animal life. This discovery makes Dickensonia one of the earliest Ediacaran animals directly identifiable in the record. This doesn’t mean it was the earliest animal, only the earliest recognizable fossil. But if the interpreted fossilized spongin networks in pre-Cryogenian rocks prove true (see the previous article on Our Common Ancestors), then Dickinsonia was a latecomer to the biosphere’s animal kingdom and the Ediacaran biota springs from a lineage much longer than 94 million years.