Microbes and DNA from 8-Million-Year-Old Ice Shed Light on Origin-of-Life Question
We have a chest freezer in our garage that stores all the food that won’t fit in our kitchen refrigerator’s freezer. When it’s time to get something from the freezer, it usually comes from the top. It’s just too much work to dig down into the chest to get to a package of frozen food. Eventually stuff at the top finds its way down toward the bottom as food gets moved around and new items are added. On the rare occasion when somebody works up the motivation to “drill down” to the freezer’s bottom, they usually encounter barely recognizable items covered with a thick coating of frost or meat with severe freezer burn. Either way, nobody wants to eat the food from the bottom of the deep freezer. The good stuff is found at the top.
A team of scientists on a quest for “good stuff” recently drilled down into Earth’s deep freezer in the Mullins and Upper Beacon Valleys of Antarctica. And good stuff they found. The researchers recovered viable microbes and DNA from ice samples dated between 100,000 and 8 million years in age. It was like finding a popsicle in the bottom of the freezer.
This remarkable discovery bears important implications for the origin of life, calling into question the likelihood of life originating in comets or being delivered to Earth by these icy vehicles. Scientists are interested in looking for microbial life in the ancient ice sheets of Antarctica because such a discovery provides insight into the limits of biological activity and the preservation of life. It also provides a record of ancient life forms and a window to Earth’s past.
Scanning electron microscopy revealed the existence of filaments in the ice samples, which are taken as evidence for the presence of microorganisms. The researchers confirmed this interpretation by demonstrating metabolic activity in the ice through the uptake of radiolabeled amino acids and nucleotides and the breakdown of radiotagged glucose.
Based on these results, they successfully attempted to cultivate microbes from the ice. The researchers noted that the best stuff was found near the top of the Antarctic deep freezer. They were able to recover more microbes and a greater diversity of bugs in the younger ice samples.
The team also searched for community DNA in the ice. To undertake this endeavor they applied genomics techniques to enable direct sequencing of DNA isolated from the ice, without isolating and culturing the individual species of microbes.
They detected, based on DNA, 30 different types of microbes in the ice. The researchers noted diminished diversity in the older ice samples (as was the case for the metabolic studies and cultivation work).
They also characterized the fragment size of DNA in the ice and, once again, noted a greater degree of fragmentation and smaller fragments in the older ice samples. Using this data, the scientists estimated the half-life of the fragmentation process to be about 1.1 million years. The rate of breakdown indicates that cosmic radiation impinging on the ice is the primary driver of DNA fragmentation.
The relative instability of DNA in the ice and the loss in the viability of the microbes over the span of 8 million years makes it unlikely that life could arise in comets or be transported through interstellar space in these frozen bodies. Comets consist of frozen water along with dust and rock. The ice in Antarctica serves as a good model for the preservation of life and durability of biomolecules in comets. In fact, the ice in Antarctica represents the best-case scenario for survivability. Even though this region of Earth receives the highest dosage of cosmic radiation, it is not nearly as harsh an environment as interstellar space.
The short half-life of DNA and the limited viability of microbes after 8 million years in the ice of Antarctica makes it difficult to envision how life could emerge in comets of survive a journey of any length between star systems or within a solar system.
Now, to defrost my freezer. I hope I don’t find any originating life in the ice.