An article published in the journal “Proceedings of the National Academy of Sciences” describes a research that provides evidence that ancient sea sponges (photo ©Yoruno) were the first animals existed on Earth. A study led by Dr. David Gold of MIT (Massachusetts Institute of Technology) reached this conclusion by analyzing an unusual molecule found in 640 million years old rocks.

The paleontological research regarding life forms preceding the Cambrian explosion are particularly complex. Over time, scientists have found many fossils from that period, between 521 and 514 million years or so, but those from previous periods make it difficult to recognize their relation with organisms that lived later.

Dr. Gold, along with Professor Roger Summons, also of MIT, and other colleagues sought some answers through a different type of analysis. The team tried to reconstruct the history of the animals based on biochemical traces, by analyzing a molecule called 24-isopropylcholestane or simply 24-ipc, a modified version of cholesterol.

24-ipc was found in rocks dating from the Cambrian period and slightly older in 1994 and the presence of this molecule was confirmed in 640 million years old rock samples collected in Oman. These biochemical traces could represent the oldest evidence of animal life so Dr. Gold’s team tried to track down the type of organisms that generated them.

The first step in the study was to identify the gene that produces 24-ipc. At that point, the scientists looked for the organisms that possess that gene. The final step was to verify how the gene evolved in those organisms.

The gene was identified by comparing the DNA from 30 different organisms including plants, fungi, algae and sea sponges. The scientists discovered a gene called sterol methyltransferase (SMT), which produces certain types of sterols according to the number of existing copies in an organism. Various species of sea sponges and algae that produce 24-ipc have an extra copy of SMT compared to their closest relatives.

Further comparisons have allowed the reconstruction of relations and the evolution of that gene to understand when it duplicated. In the resulting evolutionary tree, sea sponges evolved an extra copy of the SMT gene long before algae, about 640 million years ago, the age of the rocks in which fossils traces of 24-ipc were found.

This is an important evidence that simple sea sponges existed 640 million years ago. The hypothesis that those were the first animals appeared on Earth is not new, the results of this study are a confirmation. These sponges over time diversified to form the phylum Porifera. We don’t know how they looked because the fossil traces from that era are really limited. Sophisticated studies on fossil traces of ancient molecules may help us understand more about those primitive life forms.