Have you ever wondered how and when animals transitioned from one evolutionary stage to the next? When did animals first appear, where did they appear, and why did they appear? What were their personalities like?

For most of Earth's 4.5-billion-year existence, life has existed, but bacteria have dominated it.

Although scientists have been looking for evidence of biological evolution for almost a century, certain areas of the fossil record remain bafflingly mysterious, and uncovering evidence of Earth's first species has proven especially difficult.

Fossils

Evolution
(Photo : Dr. Guojie Zhang from the University of Copenhagen)

Fossils are the primary source of information on evolutionary events that occurred hundreds of millions of years ago. For example, shells, exoskeletons, and bones are common fossils created by living animals. These so-called "hard components" first emerged in rocks deposited 540 million years ago during the Cambrian explosion.

The emergence of a varied range of sophisticated organisms, many of which include hard parts, suggests that there was a time when early soft-bodied animals with no hard parts developed from simpler species. Unfortunately, until recently, evidence of fossil species from the "hidden" evolutionary period has been few and difficult to interpret, keeping the timing and type of evolutionary events unknown.

This problem, known as Darwin's dilemma, is still fascinating and unsolved 160 years after On the Origin of Species was published.

Related Article: Recently Discovered Sponge Fossil May be World's Earliest Ancient Animal

Weighing Evidence 

There is circumstantial evidence about how and when animals first appeared. Animals eat previous organic matter by definition, and their metabolisms require a certain amount of ambient oxygen. Thus, animals were thought not to exist, or at least diversify, until the Neoproterozoic Era, between 815 and 540 million years ago, when a huge oxygen rise resulted from the buildup of oxygen created photosynthesizing cyanobacteria, also known as blue-green algae.

Sponges are usually thought to be the most basic animal in the animal evolutionary tree, and hence were likely the first to arise. Sponges are animals because they breathe and eat by drawing water containing organic materials through their bodies. According to a genetic approach called molecular phylogeny, which studies genetic differences, the earliest creatures were probably sponge-related (the "sponge-first" theory) and may have developed hundreds of millions of years earlier to the Cambrian.

Sponges

Green and yellow sea sponges, Antarctica
(Photo : From Steve Rupp, National Science Foundation)

Sponges might have existed as long as 900 million years ago, based on these acceptable estimates. So, why haven't we discovered sponge fossils in rocks from those hundreds of millions of years ago?

Sponges do not have typical complex pieces is part of the solution to this issue (shells, bones). Although some sponges have an internal skeleton of small mineralized rods known as spicules, no credible spicules have been discovered in rocks dating from the concealed early animal evolution period. On the other hand, some sponges have a skeleton formed of strong protein fibers called spongin, which includes a unique, tiny, three-dimensional meshwork similar to that of a bath sponge.

The soft tissue of contemporary and fossil sponges has been calcified during decomposition, preserving them in the geologic record. A characteristic tiny meshwork of complexly branching tubes forms in the rock if the calcified aggregate solidifies around spongin fibers before they disintegrate. The branching pattern is distinct from that of algae, bacteria, or fungus, and it has been observed in limestones younger than 540 million years.

Microstructure 

This same microstructure was recently reported in 890-million-year-old rocks from northern Canada by a geologist and paleobiologist who works on ancient limestone, claiming that it might be proof of sponges hundreds of millions of years older than the next-youngest undisputed sponge fossil.

Although the researcher's proposal appears to be outlandish at first glance, it aligns with standard predictions and assumptions in the paleontological community: the new evidence seems to validate an extrapolated timeline and a predicted identity for early animals that are already widely accepted.

If they are sponge fossils, animal evolution might be pushed back hundreds of millions of years.

The study's early potential sponges lived alongside limited cyanobacterial populations that provided oxygen oases in an otherwise low-oxygen environment before the Neoproterozoic oxygenation episode. Before more diversified creatures arose, these early sponges may have continued to live in similar habitats for hundreds of millions of years, presumably unmodified and unaffected by evolutionary pressure.

The presence of 890-million-year-old organisms further suggests that the contentious Cryogenian glacial periods - sometimes known as "snowball Earth" - that began approximately 720 million years ago had little impact on biological evolution.

Tackling Darwin's Dillema

The rare fossil material might shed new light on Darwin's conundrum. However, radical new ideas are seldom fully adopted by the scientific community without much debate, so I expect a lot of that. Nevertheless, a consensus may emerge based on more research at some time in the future, most likely years from now. 

Also Read: Research Shows How the Newly Discovered African "Climate Seesaw" Affected Human Evolution 

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