Biological Imperatives: Reaching for Starlight

What does life do? What is life driven to do? We might not have an exact definition of life (will we ever?), but our understanding of life must be rooted in how we experience the universe, what life does, and how we can use our knowledge of science to look for other life out there.

Moments in the Sun can sometimes feel magical and rejuvenating. Standing in the outdoor air, feeling the world around us, and sensing the warmth that comes from the Sun — it can bring on an otherworldly sense of connection to our closest star.

I also get this sense to some degree when climbing trees. Moving from limb to limb, ambling through a forest within a tree to gain more vertical height. It brings us closer to the tree top, which is itself a projection of a tree that is trying its very hardest to get as close as possible to starlight.

Trees are incredible for this behavior, and I often find myself wondering: could there be alien trees out there in the cosmos?

A tree is quite literally derived from a biological drive for a plant to be able to support itself well enough so that it can grow higher and closer to the Sun — and oftentimes will even be competitive with other plant species and populations over who can get as much of that sunlight as possible.

It can be easy for many of us to take trees for granted. We see them all around us. They naturally inhabit most of the regions of our world where humans live (and some places we don’t). There are trees around us in the remaining regions of wilderness, but also within our towns and cities, sometimes even in our yards or gardens (or even within our homes). And there are lots and lots of different kinds of trees.

I honestly feel kind of ashamed of myself that I’ve never really invested the time in knowing enough botany to know just which species of trees are around the places where I have lived. That’s something I need to change.

But even in the face of my glaring lack of knowledge, I still know well enough to point out many an oak, aspen, weeping willow, birch, fir, palm, or even trees that bear apples and other common fruits. I imagine the same is quite true for many of those reading this. You probably know at least some of the types of trees that live around you (but huge kudos if you are the kind of person who has taken on the work of getting to know all of your local flora well enough to point out the name of any plant near you — you have my respect!).

But even knowing a few types of trees around us, seems like it’s still not enough for most people to really find themselves in awe of what a tree is.

Trees are incredible. They’re beautiful and varied and inspire us. Walking among trees is forest bathing, it’s meditation. But that’s not all.

On top of the fact that trees are organisms that are incredible creatures reaching for starlight, the very concept of a tree itself is not rooted within one single phylogenetic group. The various kinds of trees are not all immediately related to each other. They aren’t in a single group that sets them apart from non-trees.

Phylogenetics and the Strange Case of Trees

One thing that has been noticeable through time is that life on Earth is very connected. Not only do we know now that we all share the exact same basic biochemistry and fundamental biomolecules, but it’s pretty apparent when you start looking around at the living things that are immediately visible to us (like other animals as well as plants and fungi) that living things are also related.

We can see other primates and see our similarities. We see that there are many different kinds of birds or fish, and know that they are certainly all related to one another in their own respective groups.

There is such an incredible diversity to life on this planet, and many people as far back as we know have seen the relationships among most living things.

Grouping together living things based on their connections is known as biological taxonomy.

A taxonomy is simply a system whereby items that share some kind of connection are classified within a structure that puts them into groups or types — the word originally came from the Greek roots taxis (order) and nomos (law). Often, a taxonomy has some kind of hierarchy that reveals how things not only connect to each other but how some items within the system have grown or developed from other items.

One of the earliest examples of an attempt at a taxonomy is the Scala Naturae, or the “ladder of life”, as proposed by early thinkers such as Aristotle.

The concept of a ladder of life presents life as a hierarchy, often presuming some organisms to be “lower” forms of life while others take ever greater steps up the ladder. Mixed with some ancient religious beliefs, thinking of life as a ladder also led to many people assuming humans to be at the top of the ladder — a pinnacle to life on Earth. In medieval Christian thinking, this led to the “great chain of being”, a concept which placed a deity at the top of the ladder with angelic beings below and then humans below that — while still maintaining the place of humans above all other living things on Earth.

But some early thinkers started wondering if maybe life wasn’t just one single staircase of ascending from lower life forms to higher ones. For instance, Charles Darwin, in his On the Origin of Species, famously drew up one idea of how life might form a branching tree of connections rather than a ladder.

The biologist Ernst Haeckel took the tree idea even further, proposing several treelike structures to contain all of known life of his time, and coining the term “phylogeny” for these treelike structures that connect species to their shared histories. By the 1900s, it seemed quite clear that living things that appeared much alike also likely had a shared origin (a branching point between them on the tree of life).

In more recent times, we have learned how to use the sequencing of genes to show a biological taxonomy that derives directly from the shared evolutionary history of organisms.

This form of biological taxonomy is not based on what we look like, but comes from the genes we’ve inherited that tell our cells how to make us, and it’s known as phylogenetics.

Using phylogenetics, we can see how closely we are related to chimpanzees or fish or bananas. We also see how the evolutionary relationships among various organisms have led to certain features, like breathing underwater, having feathers, or laying eggs. But one thing you won’t find in phylogenetics is all of the different kinds of trees clumped together - there is no single branching point on the tree of life where trees themselves all have one shared origin.

There are some families of plants that have trees, some that have no trees, and some that are only trees.

This happens because the evolutionary drive to develop a woody stem and reach for starlight has happened in several different families of plants. We see the power of flight in birds and bats, even though bats are mammals, and this happens because evolution converged on this capability for different organisms. And much the same is happening among trees.

And this has me wondering: will there be an evolutionary drive for the development of tree-like organisms on alien worlds?

Alien Trees

Certainly, if evolution can converge on the structure of a tree in different scenarios, then perhaps if alien biospheres develop plant-like beings then they are also likely to face a biological imperative to develop trees. There are even examples from Earth’s past of giant mushrooms taking on tree-like forms.

There’ve been a bunch of attempts by science fiction writers and creators to consider what alien trees might be like. Avatar had its humongous Home Tree, the game No Man’s Sky includes an incredible variety of worlds with tree like beings, there are works like Ursula K. Le Guin’s The Word for World is Forest and C.S. Lewis’ Out of the Silent Planet, and the book and film Annihilation, just to name a very small few.

I recall using a VHS tape to record a TLC documentary series called Solar Empire that came out in 1997. The show was narrated by Michael Dorn of Star Trek fame, and included a lot of topics in space exploration for my young mind to fathom. Among the episodes was one where they highlighted a project called Epona — it was a world building exercise to consider the evolution of life on a hypothetical exoplanet. Among their attempts to sculpt out how one possible biosphere might grow from its origins to a diversity of life similar to our own, they envisioned what some alien trees might look like.

As Carl Sagan discussed in the show Cosmos when considering the potential for alien life on a Jupiter-like planet, there is currently no known law of biology that guides our knowledge of what is probable for alien life forms, but we can use our knowledge of chemistry and physics to at least help us better understand what is possible. Some scientists have already been speculating based on our current knowledge what the organisms of other worlds may be like, including plants and tree-like forms.

What I’d like to pose here for your own consideration: do you think it’s likely that life on alien worlds will be driven to develop trees?

Maybe some biospheres have evolved different kinds of structures that allow creatures to reach out for more starlight. Perhaps what we know as flight here is only one small taste of the ranges of creatures taking to the air — could there be giant balloon-like creatures that get higher and higher to get more starlight? We could probably sit and conjure up our own alien worlds with their own biological histories that have allowed for an assortment of tree-like forms but also others.

Still, trees are simply remarkable, and I’d be willing to bet that we’ll one day find out that there are many of them “out there”.