In a previous article, we established that the probability is high that there is life elsewhere in the universe in our article: Is Earth the only planet with life in the Universe?. But the question is, will those life forms be like us? Because our technology is not yet at a level that permits us to go explore the other planets where there is high likelihood of life, let us use Earth as our test subject. What is the most prevalent form of life on our planet?
Well, for starters, the most prevalent life form is not humans. Not even close. On Earth, the long-standing champion for most abundant life form was the bacteria group called Pelagibacterales (SAR 11).
SAR11 Clade Dominates Ocean Surface Bacterioplankton Communities
These are single celled bacteria that live in the ocean and eat undissolved organic carbon and nitrogen. There are estimated to be 2.4×10^28 Pelagibacterales on the planet earth. Better said, the population of this bacteria is 24,000,000,000,000,000,000,000,000,000. There are more of these bacteria on Earth than there are stars in the observable universe…. And a lot more, not just like 6 more, but like thousands of times more.
We tend to forget about bacteria as a life form because we don’t see them as we’re walking around each day. But bacteria own this planet. Scientists have found evidence of bacteria fossils from hydrothermal vents that are somewhere between 3.7 and 4.2 BILLION years old.
Evidence for Early Life in Earth’s Oldest Hydrothermal Vent Precipitates
To put this into perspective, Earth is estimated to be about 4.5 billion years old. Trees, meanwhile are only estimated to have evolved about 350 million years ago. This means that some bacteria are almost as old as Earth itself, with trees being just infants in comparison on the time continuum. But as we’ve discussed before, sometimes numbers of this size are hard to visualize. Let’s look at this another way:
Let’s say that the entire history of the Earth, from formation until today, fits into one 24-hour period starting at Midnight. The earliest bacteria show up around 1:30 in the morning. The first multicellular organisms show up around 8:48 AT NIGHT. Trees first evolve a few hours later, around 10:08 pm and the Pyramids at Giza were built about 8 milliseconds before the end of day.
Bacteria have been the dominant lifeform on earth for a very long time. Bacteria were likely the only life on the planet for billions of years. Humans and most of the bits of our environment that we are used to are very, very new to the party. Even within our own bodies, we are not necessarily the dominant life form by number.
What does that mean? Well, an average person is estimated to be structurally composed of around 30 trillion cells. That same person’s body is estimated to be home to about 39 trillion bacteria. If you just took a completely random cell from your body, there is about 50% chance that the cell would be a bacterium and not actually contain any of your DNA. We’re basically giant walking condos for bacteria to live; humans are just a nice, warm, moist home.
Very recently there have been some recent developments in the studies of most abundant life on this planet and the long standing champion SAR11 may have been dethroned. Its usurper is actually not only an even smaller form of life but one that lives off the Pelagibacterales! The new king of the population empire on Earth is a virus called HTVC010P.
Abundant SAR11 Viruses in the Ocean
This virus, by sheer number, is likely the most abundant on earth. Viruses are very, very small so they escaped previous studies trying to find the most abundant life.
Viruses may have evolved shortly after bacteria, but evidence of this evolution is harder to find. We have evidence of bacteria evolution from fossil records. Viruses though are much smaller and harder to find in via that method, so we don’t have definitive proof of when they first showed up on earth. Our only evidence is based on genetic sequencing and common ancestor estimations.
So perhaps viruses are more common on earth than bacteria, but then you must be asking yourself, is a virus really alive? What is life? (definitions are key to a good scientific question as we’ve covered here) Depending on how you define life, a virus might not be considered living organisms in the same way that we define life in other organisms (but we’ll get to that in a later article).
Either way, based on the numbers, statistically it is very unlikely we are the only lifeform in the universe. There is most likely at least some other life forms out there in the universe and based on the life on Earth, bacteria or viruses are statistically the most likely life we’ll find out there in the cosmos.
Further info:
Citation for Journals:
Dodd, MS, Papineau, D, Grenne, T et al. (5 more authors) (2017) Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature, 543 (7643). pp. 60-64. ISSN 0028-0836
Yanlin Zhao, Ben Temperton, J. Cameron Thrash, Michael S. Schwalbach, Kevin L. Vergin, Zachary C. Landry, Mark Ellisman, Tom Deerinck, Matthew B. Sullivan & Stephen J. Giovannoni. Abundant SAR11 viruses in the ocean. Nature volume 494, pages 357–360 (21 February 2013). doi:10.1038/nature11921
Morris RM, Rappé MS, Connon SA, Vergin KL, Siebold WA, Carlson CA, Giovannoni SJ. SAR11 clade dominates ocean surface bacterioplankton communities. Nature. 2002 Dec 19-26;420(6917):806-10.