Evidence of ancient life on Earth is scattered around the globe. The most obvious examples of past life include visible fossils, preserved in sedimentary rocks in the geologic record, viewable in museums and paleontology laboratories alike. But the visible fossil record doesn’t fully reflect the diversity of life on ancient Earth — for that, researchers focus on the microscopic. They also search for molecular fossils, compounds that resist significant biological and thermal degradation on geologic timescales.
This past Monday, researchers at the University of California (UC), Riverside, identified evidence for some of the earliest animal life on Earth using molecular fossils diagnostic for multicellular animals called demosponges, which encompass over eight thousand different species of sponge — including the common bath sponge. Their findings were published in the journal Nature Ecology & Evolution.
Roger Summons, the Schlumberger Professor of Geobiology in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) co-authored the paper along with scientists from California Institute of Technology, Geoscience Australia, Central Michigan University, Uppsala University and Stanford University.
The researchers identified a previously unknown sterane molecule (a degraded and saturated steroid), called 26-methylstigmastane, in rocks sampled from the Huqf Supergroup of the South Oman Salt Basin, a particularly well-preserved sedimentary sequence with extensive sample availability due to the long history of petroleum exploration there. The ages of the rocks span the Cryogenian through early Cambrian periods — from over 635 million years ago to approximately 540 million years ago. The chemical structure of this sterane was determined by rigorous comparisons with the carbon skeletons of sterols present in some modern-day demosponges.
The research was first-authored by graduate student J. Alex Zumberge in the UC research group of Gordon D. Love, a former EAPS postdoctoral research fellow from 2003 to 2006. While at MIT, Love researched molecular fossils from the Proterozoic eon as well as from the end-Permian mass extinctions in the Summons Lab.
For a number of years now, Love has been a strong proponent of the sponge marker hypothesis: the hypothesis that another molecular fossil, sterane 24-isopropylcholestane, can be a diagnostic tool for sponges. But evidence of 24-isopropylcholestane has also been found at trace levels in some algae today, complicating its use for tracking sponge evolutionary history. Love’s identification of 26-methylstigmastane doesn’t have that problem: its sterol precursor is only found in demosponges.
“He has continued to find creative ways to test the sponge biomarker hypothesis,” Summons says, “and now, with his student Alex Zumberge and colleagues with expertise in sponge biology, has discovered a carbon skeleton in ancient sedimentary rocks that is unique to demosponges.”
Through their research, the researchers estimate that demosponges were ecologically prominent in marine environments well-prior to the Cambrian explosion, known for its rich fossil records of diverse animal clades. These estimates line up with current molecular clock predictions, which make use of the genomic histories contained in the genes of living organisms and their ancient fossil counterparts.
“The sponge biomarker hypothesis will continue to be controversial because presently, it cannot be reconciled with the record of fossilized sponge spicules,” Summons says. Spicules, structural support elements in sponges, are also preserved in the fossil record for some demosponges — but none so far date back to the ages in which the 26-methlystigmastane was found. However, a 2010 study suggests that siliceous sponge spicules existed in the Precambrian — as the presence of 26-methlystigmastane would suggest — but were not preserved. “Ultimately, the outcome of this work shows the power of persistence and the scientific method,” Summons says.
This research was supported, in part, by the NASA Astrobiology Institute (NAI), Foundations of Complex Life based at MIT, NAI’s Alternative Earths, NASA Exobiology, the National Science Foundation’s Frontiers in Earth Systems Dynamics, as well as the Agouron Institute and the SponGES project.