What Is Life?

Trigger warning for people who have been traumatized by or become anxious around animals.

Adalbert Dragon/Shutterstock

Life is incredibly precious, you realize seconds after meeting a cat like this and knowing that you might come out on the losing end here.

The usual way we see our lives — as something to get through each day — evaporates under this jaguar’s frank stare.

Welcome to the food chain, pal!

Like it or not, you and I are part of the great web of life.

Individually and as groups, we try to avoid the scary and unpleasant parts by insulating ourselves from Nature as much as possible. This often works, too.

However, Nature is bigger than us. Bottom line: like any other species, H. sapiens eats and can be eaten.

Now for the good news.

“Cave lions suck!” “Bears, too! — Prehistoric people. (Image:
EOL, CC 2.0)

Human beings have been dealing successfully with predators like this jaguar and even worse for hundreds of millennia.

The survivors of such encounters have passed along to us a built-in emergency mode that gives our famously big brains a chance to think their way through a crisis.

Of course, since we live in a very different world from theirs, such responses sometimes do us more harm than good.

That’s rough for some people, but evolution works on groups, not individuals. (Fitch and Ayala; Simpson) It’s going to take a little more time for humanity to evolve better ways to deal with new kinds of stress.

An internal dialogue

Adalbert Dragon/Shutterstock

Meanwhile, there’s this jaguar.

Stress hormones surge and your brain immediately goes into overdrive:

  • Did I lock my car, out in the parking lot?” (Don’t panic. Focus.)
  • “Okay. Exactly how hungry is Life feeling at the moment?” (Can’t tell.)
  • “What’s its mood?” (Please. It’s a cat — who knows? But it’s definitely interested in you.)
  • “I can see that. Where’s the nearest tree?” (Over there, and it’s got a low-hanging big branch, you descendant of primates. But…)
  • “That’s enough. Can this cat climb?” (Unlike, say, a lion, jaguars can climb quite well, and evolution has also made them good sprinters. Note, too, the powerful and well-muscled body — “Will you shut up!!! — I was just pointing out that you can’t fight this beast; flight is your best chance. In any encounter with a cat big enough to harm you, just move away slowly. Don’t turn your back on it. If nothing else, each backward step gives you more of a head start. But…)
  • “Not now. All righty, I am now taking the first step back with one foot and — AAAGH! WHAT’S THAT!!! (Just the janitor’s cart, Tarzan. I was trying to tell you. You’re in the big cat pavilion at the city zoo. Your subliminal mind picked up those window reflections in the jaguar’s eyes and was trying to point out that you’re indoors.) “‘Car keys’ and ‘indoors’ are two different concepts! (Must have made sense subliminally.) You fell for it at first, too, Rational Voice In My Head! (Forget it. All that nervous energy you just burned needs to be replaced. Let’s find a food vendor.)

And off you go, continuing the inner dialogue that all of us sometimes have with ourselves.

But the jaguar continues to stare as you head towards the door (after you’ve apologized to the janitor, who seems amused: apparently other people get that primitive reaction to their first close-up view of a big cat, too).

This apex predator totally understood your instinctive fear, Monkey Person, and was ready to respond to any move you might have made. Its ancestors have passed down helpful genes, too.

Life is a little like an arms race that way. No one ever holds the advantage for long.

What this big cat does not understand are things like a protective glass panel to keep both visitors and animals safe, or what a zoo is, or firearms and snares, or why Monkey People are sometimes cruel, sometimes merciful.

Whatever. Since its evolution emphasized muscles over brain power, the jaguar quickly loses interest after you’re out of sight and eventually wanders off for a nap.

While dozing, perhaps it says to itself, “Funny old thing, Life. No matter where you are, whenever you look at it, there it is, looking back at you.”

Defining “life”

So, life is — genes? DNA?

Weeeeeelllllll. Think about that jaguar encounter. Could “polynucleotide chains”(as Dr. Wikipedia currently defines DNA) produce all that?

Just what is a nucleotide, anyway, and what does it have to do with jaguars, the world around us, and our own complexity?

The shortest, simplest answer I can give you is this: go get your PhD in molecular biology and apply for a grant, because all of this involves intense biochemistry and also is a very hot research topic right now.

There are more questions than answers, too.

Experts have learned a lot during their studies, but they still don’t know what Life is — other than that it usually involves, in each cell:

  • Molecules like DNA and/or RNA.
  • A variety of other molecules and tiny structures
  • Some incredibly complex biological mechanisms that copy the cell, check the duplicate one, and correct any errors they find.

Valentina Kru/Shutterstock

Cells are very complicated. Over geologic time, and with the right “game rules” (using our cat-on-a-sports-field analogy again), it’s quite possible that cells really are building blocks of a complex, diverse world.

Scientists are convinced of it, anyway.

For our simple purposes, let’s just call that intracellular material “stuff” whenever possible. Our questions, after all (including the one behind this series: in simple terms, how did cats evolve?), these all have to do with the world outside the microscope.

Anyway, while the boffins are very knowledgeable about the “stuff,” they still can’t agree on what Life is.

At least a hundred scientific definitions exist, but almost every one of them is different in important ways from the others. (Trifonov; Walker et al.)

Trifonov did a linguistic analysis of those definitions and found the most frequently used words, strung together, say, “Life is self-replication with variation.”


“Variation” is just another word for “mutation.” As I understand it, this happens when a cell’s QA systems miss something while checking the duplicate cell for errors.

This mutation can be harmful, or it might not have any immediate effects. Sometimes, though, mutations make offspring slightly different from their parents.

We’re talking small differences here: it’s just cells, after all.

Individuals only physically change before your very eyes in movies. (But cell mutations can cause diseases like cancer.)

Evolution happens when there is a population, such as a flock of avian dinosaurs (a/k/a birds); reproduction of the mutation; interactions between the mutation owners and their environment/other forms of life; and geological spans of time.

Or not.

“We’re not lazy, just contented.” — Stromatolites. (Image: Pat Scullion, CC BY-ND-NC 2.0)

The oldest known fossils, some 3.5 billion years old, are almost identical to their modern descendants: today’s cyanobacteria (the critters formerly known as blue-green algae, found today as stromatolites and also living in other places). (Doolittle and Brown)

Still, one glance around does show that a heckuva lot of evolution has gone on in most other living beings since Precambrian days.

But whenever the best minds on our planet look closely at this, digging through all that cellular “stuff” in people, animals, plants, and so forth, they always find — not the ultimate answer but Life, in the form of even more cellular “stuff,” looking back at them inscrutably and without any self-explanation whatsoever.

It must be frustrating for all those PhDs.

The last universal common ancestor (LUCA)

Walker et al., in the source list, note that one way around this problem is to go back to Life’s beginnings.

This approach has occurred to other great minds, too.

Everything presumably was more simple back then, so the transition from inert to living material theoretically might be easier to spot.

You might be wondering if any fossils survived Earth’s earliest days (the first billion years or so), which were tumultuous — maybe.

Not that we know of. The planet is roughly 4.6 billion years old, and no fossils older than 3.5 billion years are recognized.

Some experts also point out that the Precambrian cyanobacteria we do know about already had very complex “stuff.” (Doolittle and Brown)

In other words, something more primitive probably existed before cyanobacteria could evolve, but what?

Science was pretty much stuck at this point until two major breakthroughs happened:

  1. The discovery of underwater hydrothermal vents that support all sorts of extremely durable and flexible forms of primitive life.

  2. Identification of “molecular fossils.” NOT fossilized molecules: while these are very cool, as well as informative, they don’t go back far enough in geological time to help us define Life.

    Here is Maizels and Weiner’s definition: “A molecular fossil is any molecule whose contemporary structure or function provides a clue to its evolutionary history…it records, embodies, and reflects ancient evolution but is not itself ancient.”

    Things quickly dive into biochemistry and other arcane matters after that, but apparently such things can be found in cellular “stuff,” and they do provide clues to life’s ancient beginnings.

To make a long story short, and to get set for the next chapter, let’s just say that at the moment paleobiologists suspect that life got started at deep hydrothermal vents early in the planet’s history, even before conditions at the surface were tolerable.

Too, all that technical work trying to define life by studying “stuff” has led to the idea of there being three domains of life (and maybe also viruses, but they’re very weird and we should save those for the next chapter).

For reasons that have to do with “stuff,” everything you physically are, and everything you can see without a microscope, can be classified as part of the eukaryote domain. Microscopic critters that aren’t viruses belong in either the bacteria or archaea domains. (Doolittle and Brown; Maizels and Weiner; Walker et al.)

Don’t worry. There’s no quiz.

What’s really interesting is that, while sorting through the cellular “stuff,” boffins have found some vitally important biochemical similarities among all three domains. (Maizels and Weiner)

Mike Goad, public domain.

In other words, they say that these three groups — now looking as different as a leaf, a jaguar, and some colorful microbe basking in the hot waters of Yellowstone’s Grand Prismatic Spring — all had a common ancestor — LUCA, or “last universal common ancestor” — back in the days when Earth was young.

We must be getting close to the start of it all, close to actually being able to say what Life really is!

But guess what? Walker et al. write that LUCA was already “a highly evolved cellular life form, complete with genetic translation machinery.”

In plain language, something or multiple somethings had to exist so that the cellular “stuff” that molecular fossils are made of could evolve.

And, without evidence from those pre-LUCA life forms (because molecular fossils hadn’t evolved yet), we literally have no clue what that primordial material was. (Walker et al.)

But we do know it was alive, because LUCA’s cellular “stuff” did evolve.

Yeah, it’s confusing. Look at it this way:

  • You need living things for evolution to work on, right? Otherwise, it’s all just chemistry and physics processes — take H2O, for instance. It does many things, but it never evolves on its own.
  • Well, everything geoscientists know or have theorized about the physical conditions of Earth’s formation — basically, heat and pressure that would put a modern thermonuclear blast to shame, happening over and over again — all of this rules out the possibility of Life being part of the Earth v. 1.0 package.
  • As far as we know.
  • Life exists on Earth now, so it had to start at some point.
  • LUCA was not that point, per experts in ancient life whose work I have read.

Sigh. Life is so very, very much older than us.

No matter how far back we go, trying to discover what it is, we find Life already in existence, staring back at us.

Of course they’ve tried lab experiments and computer modelling, too. This has been going on since at least the 1950s.

But all anyone, even NASA in recent times, has been able to do is to synthesize simple biomolecules, nowhere near the complexity of even the most basic living cell. (Walker et al.)

It would certainly help to really know what ancient Earth’s geochemistry was like, and what sort of biomolecules it could produce.

But, what with billions of years’ worth of factors like plate tectonics, weathering, and the many physical effects life has on both land and sea, direct geologic evidence of our planet’s birth and toddlerhood is long gone.

However, the whole Solar System did form at the same time as Earth and there are extraterrestrial rocky (and icy) archives from those ancient times that are still well preserved.

If we can get out there to them, is it possible to read their record correctly? Would we find more clues about our own beginnings?

This is perhaps the only scenario where it truly would be wonderful to find Life staring back at us.

Unless it attacked, of course. But that’s probably just a movie thing, too.

Right, NASA? Uh, NASA? SETI? Captain Picard? Greenpeace?

Wheeee! We’re having too much fun to come to the phone.

Edited June 24, 2021.

Featured image: Adalbert Dragon/Shutterstock


Doolittle, W. F., and Brown, J. R. 1994. Tempo, mode, the progenote, and the universal root. Proceedings of the National Academy of Sciences, 91(15), 6721-6728.

Durzyńska, J., and Goździcka-Józefiak, A. (2015). Viruses and cells intertwined since the dawn of evolution. Virology journal, 12(1), 1-10.

Fitch, W. M., and Ayala, F. J. 1995. Preface. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington: National Academy Press.

Maizels, N., and Weiner, A. M. 1994. Phylogeny from function: evidence from the molecular fossil record that tRNA originated in replication, not translation. Proceedings of the National Academy of Sciences, 91(15), 6729-6734.

Niklas, K. J. 1994. Morphological evolution through complex domains of fitness. Proceedings of the National Academy of Sciences, 91(15), 6772-6779.

O’Donnell, M., Langston, L., and Stillman, B. 2013. Principles and concepts of DNA replication in bacteria, archaea, and eukarya. Cold Spring Harbor perspectives in biology, 5(7), a010108.

Schopf, J. W. 1994. Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic. Proceedings of the National Academy of Sciences, 91(15), 6735-6742.

Simpson, G. G. 1944. Tempo and Mode in Evolution. New York: Columbia University Press.

Walker, S. I., Packard, N., and Cody, G. D. 2017. Re-conceptualizing the origins of life. Philosophical Transactions of the Royal Society A, https://royalsocietypublishing.org/doi/full/10.1098/rsta.2016.0337 .

Wessner, D. R. 2010. The origins of viruses. Nature Education, 3(9), 37. https://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218/

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