Life on Earth started billions of years ago, but early animals could have been less complicated than we think. A new study reveals that primitive sea sponges could have been among the planet’s earliest creatures, emerging hundreds of millions of years before complex life appeared.
When Did the First Animals Exist?
The planet Earth came into existence about 4.5 billion years ago, and straightforward microbial life could have emerged between 4.3 and 3.7 billion years ago. But animals took much longer to arrive. Scientists now believe sponges existed over 541 million years ago, long before most multicellular organisms. Their discovery sheds light on one of biology’s biggest questions: when did animal life truly begin?
What Evidence Links Sponges to Early Animal Life?
Scientists discovered "chemical fossils" in ancient rock samples with 30-carbon steranes, a stable type of sterols found in the cell membranes of complex organisms. These steranes are closely tied to a class of sea sponges called demosponges, suggesting they left behind molecular traces of early animal life.
“We don’t know exactly what they looked like, but they were soft-bodied ocean dwellers without silica skeletons,” said Roger Summons, a geobiologist at MIT and study author.
Why Are Sponges Classified as Animals?
Although they resemble plants, sponges are animals. They consist of many eukaryotic cells that eat, give birth, and react to the environment. They do not have cell walls or complex organs, but despite this, they qualify as essential animal criteria. Scientists think that their simplicity renders them important to discerning how animal evolution began.
How Does This Relate to the Cambrian Explosion?
Shortly after sponges evolved, the Earth went through the Cambrian explosion, where complicated species evolved and diversified in a short time. Before this event, life was mostly simple and blob-like. The presence of 30-carbon sterols in ancient rocks indicates that animal life may have begun far earlier than this evolutionary burst.
How Did Scientists Confirm the Findings?
The researchers studied rock samples from Oman, western India, and Siberia and found sterane signatures associated with demosponges. They then created 30-carbon sterol in the laboratory by using an enzyme coded by genes from sponges, establishing that the molecules were probably made by ancient animals.
“It’s a combination of what’s in the rock, what’s in the sponge, and what you can recreate in a lab,” Summons said. “All three lines of evidence point to sponges being among the first animals.”
The study, published in the Proceedings of the National Academy of Sciences, offers one of the strongest signs yet that Earth’s earliest animals were simple sponges, quietly shaping the course of evolution long before complex life took hold.
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