When we think about possible life in the universe, we soon encounter numbers. There’s too many of them, both large and small. We simply can’t grasp how huge or how small they are, especially the numbers of microbes
that may be lurking in many different places in our solar system.
The nearest star in our galaxy of 100 billion stars is about four light years away. That’s 252,600 astronomical units. Voyager is cruising along at 3 au/year, so it would take 84,200 years to get to our nearest stellar neighbor, whether or not it had any hospitable planets. Such distances are hard to imagine until we get real about the time it would take us to get there, even at a reasonable fraction of the speed of light. And there are how many other galaxies? 100 billion or so? And the nearest one is how much farther than the nearest star in the Milky Way?
To get a better idea about large numbers, consider the human brain. Science News November 30, 2013 tells us we have 86 billion neurons in our brain. (There are also three types of Glial cells that nourish, insulate and protect those neurons, but we’re not counting those.) Each neuron can have as many as 10,000 connections to other neurons. 10,000 x 86 x 10 billion is a huge number of connections, which is why students of complexity say it is the most complex object in the universe. Galaxies with their 100 billion stars are simple in comparison. No wonder we get into such trouble.
In thinking about life out there, let’s start with ourselves. Who do we think we are, compared to who we think might inhabit outer space? When I was studying medical microbiology , we focused on a few viruses that were causing trouble—polio ,flu and the common cold, hepatitis, rabies and some others. Now, suddenly we have discovered that 67.7% of the DNA sequences found in our blood plasma belong to viruses. Only 3.6% are human and 9.5% are bacterial. The rest are a mystery. (See Popeorgiev et al in Journal of Infectious Diseases 2013.) We are loaded with microbes.
We carry around lots of common disease viruses, plant disease viruses, and bacteriophages. The latter are viruses that infect the huge numbers of bacteria that inhabit our bodies. They help determine which bacteria will populate our guts. Phages mutate rapidly, and bacteria can use them against other bacteria or against us if they help the nastier bacteria make toxins or resist antibiotics.
The point of all this is that our resident microbes form a wild microworld we couldn’t live without—which should give us a notion how complex life on other moons or planets could be. Most likely, most life out there will be microbes like viruses.
Small and simple critters probably evolved first, if we suppose that simple design had to precede organisms more complex. Hence viruses—like other microbes—could be the most common type of life on Enceladus or Europa or the billion or more Earth-like planets in our Milky Way galaxy.
Viruses are very small packets of clever protein wrapped around DNA. Bacteria are huge in comparison and much more complex, with their tiny organelles, cilia and other contraptions like cell walls. But they have no nucleus, as do the much larger cells that make up our body tissues. By the way, bacteria outnumber the cells in our body by ten to one. They reproduce on their own, unlike viruses, which rely on cells or bacteria to produce their DNA offspring. Viruses outnumber the bacteria in our body ten to one.
There are 10 quintillion (1018) viruses, more or less, on Earth. And they’re everywhere. We depend on them for all kinds of physiological reasons. So does all life on Earth, probably. If life got a foothold on some other moon or planet in our solar system, we shouldn’t let it surprise us with its versatility, its tinyness and its huge numbers.