We owe cyanobacteria our respect. They have done a great deal for us. Even though they have one nasty trait that I’ll mention later.

One reason to respect them is that they are our elders, by a long shot. Cyanobacteria date back 3.5 billion years, almost to the earliest signs of life. Another reason is that they are–well, interesting. They are single-celled, though many live connected to each other in colonies and filaments. And they are primitive; unlike the cells of younger species that followed them, cyanobacteria have no nucleus. And yet they have thrived, big-time. Their species number at least two thousand that have been described and as many additional species as well. Most cyanobacteria are blue-green—“cyan” (no relation at all to the poison, cyanide)—but their various pigments also account for the colors of pink flamingoes and the Red Sea.

But here’s why they should get our respect on top of that. Without them we wouldn’t be here, because cyanobacteria gave us oxygen—and they continue to do so. For the first two billion years after the earth’s formation 4.5 billion years ago, the atmosphere contained almost no oxygen. But the blue-green pigment in cyanobacteria is a mix of green chlorophyll and a blue pigment both of which turn sunlight and carbon dioxide into the sugary energy needed by the cell. There is a waste product from this process: oxygen. And so much oxygen, produced for so many millions of years that it accumulated in the atmosphere and came to support the formation of much larger, complex cells, like ours.

Cyanobacteria also made plants possible and became an integral part of them. We see them today as the greenery of plants—the chloroplasts, a close relative—that power plants and keep them reaching for the sun.

Cyanobacteria often go by the name of blue-green algae. But they’re not algae. Algae is an informal term for many water-borne organisms that contain chlorophyll but lack stems, roots, or leaves. Seaweed is algae. Cyanobacteria are bacteria—simple cells, often strung together, without nuclei.

I said there was one nasty trait: they also form cyanotoxins that can kill you. Especially in freshwater ponds and lakes, blooms of cyanobacteria looking like blue-green paint slicks may be toxic to nerve and liver systems, depending on the species. The poisons may work their way into the food chain, pets may eat them, water-skiers may absorb them. The result can be respiratory failure, Parkinson’s, ALS. Powerful stuff, for better and worse.

Despite the dangers, we are in their debt for the breath we take, the food we eat, for our living on solid ground. We stand on their countless, tiny shoulders.