If you look at an evolutionary diagram, you can see where Homo sapiens branched off from other primates. You can see where apples branched off from the rest of the rose family. But you won't see where trees branched off from other plants. That's because they never did.

Trees are not a species, or even a family or an order. So do trees even exist, scientifically?

Tom Kimmerer, a consulting forest scientist based in Kentucky, said yes — just not in the way you might think.

"Many years ago I asked my daughter Larkin, 'What's a tree?'" Kimmerer told Live Science. "And she could recognize them without fail. If a kid could recognize it, then a tree is some real entity we can recognize, even if the definition is difficult."

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Kimmerer said a tree is simply a solution to a problem, and that problem is access to light.

The earliest plants on land were herbaceous — soft, non-woody plants like herbs and grasses that generally grow low to the ground. But the moment one plant grew taller than the others, there was competition for sunlight.

"The attribute of being a long-lived — that is, perennial — plant that keeps its leaves above the leaves of the rest of the plants is what leads to a tree," Kimmerer said. "All the other things that happen to trees have to happen in order for that to work efficiently."

For example, trees have thick trunks to support the weight above them and help them reach greater heights, and therefore more sunlight. Trees also have an efficient plumbing system to draw water up that high and bring nutrients back down — that's thanks to xylem and phloem, the tissues that form a tree's rings. Trees also have large, efficient root systems to supply all that water.

Herbaceous plants have many of these features too: they have stems to support their weight, they have xylem and phloem to carry water and nutrients, and they have root systems. These features just don't have to do as much as they would in a plant that's several hundred feet tall.

Herbaceous plants also aren't made of wood — and wood makes all the difference.

"Wood is one of the most amazing structures in the world because it's an adaptation that simultaneously solves multiple problems," Kimmerer said.

Wood does three things that are absolutely necessary for a tree: it's firm and flexible enough to support a tree's height, it's great at transporting water a long distance, and it's great at storing sugar. Without wood, trees would buckle under their own weight and struggle to get enough water and nutrients.

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But as similar as their characteristics are, trees aren't all related to each other. They're just a habit, or a form that a plant can take. You can find trees all over the evolutionary map.

"There are lots of [plant] families that are only herbaceous," Kimmerer said. "There are even more families that have both herbaceous and tree members. And then there are a few families that are only trees."

"If I were to show you a picture of a thousand different species of trees, you'd go, 'Wow, they're all different,'" Kimmerer added. "Just like we're all different. But their fundamental attributes are the ones that make a tree."

Originally published on Live Science.