Zinc-limiting view of evolution upended by UC Riverside-led research


RIVERSIDE — An unexplained delay in the evolution of eukaryotes -- creatures with a cell nucleus like humans -- wasn't due to a lack of zinc, as one theory holds, according to a study led by a team of UC Riverside researchers.

To the contrary, zinc was as available in the ancient ocean as in modern times, according to the study, which analyzed sedimentary rocks as old as 2.5 billion years ago.

The study was published Dec. 23 in Nature Geoscience . The international collaboration received funding from many sources. In the U.S., funding came from the National Science Foundation, the NASA Astrobiology Institute and the Agouron Institute in La Jolla.

This is the latest study from the researchers, who have sought to understand the chemistry of ancient oceans and how it relates to evolution and extinctions.

Eukaryotes includes all animals and plants, and many single-celled microbes. Their development was limited in structure and function before 800 to 600 million years ago, according to the fossil record. Zinc is important to eukaryotes; molecules called zinc-finger proteins are an important target for drug research.

The use of zinc marks a "significant delineation between prokaryotic and eukaryotic life," the study stated. "... Nearly 30% of known Zn-binding protein structures are found exclusively in eukaryotes and account for the entire proportional difference between prokaryotic and eukaryotic Zn-binding proteomes."

One theory is that the availability of zinc was limited in the early evolution of eukaryotes. The theory states that much of the ocean had low levels of oxygen and high levels of sulfur, a chemistry that would have made the zinc existing in the water less available to life.

Much of zinc chemistry is relatively new to eukaryotes, evolved during the last third of their existence, or about 600 million years ago. (Eurkaryotes arose about 2 billion years ago). According to the theory, eukaryotes began using more zinc when it became available, enabling their diversification.

But the new research indicates that zinc was available to life and present in oceans at concentrations similar to modern levels from 2.5 billion years ago to 542 million years ago.

The researchers analyzed marine black shale samples from North America, Africa, Australia, Asia and Europe, according to a UCR press release on the study. These samples ranged in age from 2.7 billion years to 580 million years old. The samples indicated high levels of zinc availability, and that zinc concentrations were similar to those of modern times.

“We argue that the concentration of zinc in ancient marine black shales is directly related to the concentrations of zinc in seawater and show that zinc is abundant in these rocks throughout Earth’s history,” said Clint Scott, the first author of the research paper and a former UC Riverside graduate student, in the press release. “We found no evidence for zinc biolimitation in seawater.”

UCR professor of biogeochemistry Timothy Lyons was the paper's senior author.

The study concludes that the delayed diversification of eukaryotic life and use of zinc was probably due to other unknown factors that made the use of zinc desirable at that time and not before:

"The black shale record of Zn enrichments suggests that the history of biological Zn utilization and, more broadly, metallome (metalloproteome) characteristics within specific lineages, mirrors functional requirements and evolutionary contingencies rather than environmental metal availability as previously proposed," the study stated.


