For 99% of hominin history, we were living in the Stone Age. It began some 3 million years ago, followed much later by the Bronze and Iron Ages, at just 10,000 and 5,000 years ago.

Hominins have used wood throughout those ages. For all that time, you could say we’ve been in a Wood Age -- though scientists never officially named one.

Wooden artifacts don’t often turn up in the archaeological record, because they decompose.

Nonetheless, traces have been found. Some stone tools in East Africa, dating to around 1.5 million years ago, show residue of being attached to wooden handles.

A wooden plank apparently polished by humans was dated to nearly a million years ago in Jordan. In Eurasia, wooden spears from more than 300,000 years ago were likely used for hunting or fishing.

A recent find near Kalambo Falls, in today’s Zambia, revealed wood logs cut by stone tools to form a platform or shelter. At 476,000 years old, these are the earliest known examples of wooden construction.

And wood is still widely used today. The average American uses two pounds of wood each day, mostly in packaging. Building the average American house requires two to three acres of forest! And nearly a billion people still burn wood for energy.

Though we never had an official Wood Age, we’re definitely still in it

Two of the most important geologic features of our planet, that shape our oceans and continents, are Earth’s two types of crusts – oceanic and continental.

Both types float on Earth’s asthenosphere – its semi-molten mantle.

Oceanic crust is thin, averaging just 5 miles thick. It rises out of the mantle at mid-oceanic ridges and sinks below continents at subduction zones.

It’s constantly being recycled like this, and is therefore relatively young. Our most ancient oceanic crust is just 180 million years old.

But the defining quality of oceanic crust is that it’s dense. The minerals that form it are heavier than those of continental crust, meaning it floats lower on Earth’s mantle. The average elevation of oceanic crust is 16,000 feet below sea level.

Continental crust is less dense, so it floats higher. When it contacts oceanic crust, it tends to ride up over it, pushing oceanic crust under and back into the mantle.

As a result, continental crust is recycled much less frequently. It’s much older, piles up much higher, and is 10 times thicker than oceanic crust in places.

Continental crust forms Earth’s land masses, from sea shores to mountain ranges, where land-dwelling creatures live. Without it, Earth might have only sea life.

Earth’s two mobile crusts are unique in our solar system, geologic features that shape life as we know it.

Oxygen. Without it, life on Earth would look very different. Because only plants and a few types of microorganisms can live without it.

Fortunately, plants originated 3.5 billion years ago. They inhaled so much carbon dioxide and exhaled so much oxygen that they changed Earth’s atmosphere. With more oxygen in the air, Earth could sustain animal life -- which would eventually become us.

Those first plants looked a lot different than you might imagine. They were phytoplankton, single-cell algae adrift in the surface layers of oceans. The largest were just one millimeter in diameter, and many far smaller.

Those same kinds of phytoplankton still exist today, in massive quantities, in every ocean. They were the base of the marine food web then and still are today. Without them, ocean ecosystems would collapse.

And, they still provide a great deal of Earth’s oxygen.

Just one type of phytoplankton—called cyanobacteria, or blue-green algae—makes a whopping 20% of our oxygen. That’s more than all of Earth’s rainforests put together!

Another 30% comes from other kinds of phytoplankton and marine plants, meaning fully half of Earth’s oxygen comes from the ocean. Every other breath you take!

We talk a lot about ocean health on EarthDate, and this is another reason why. The ocean and its trillions of microscopic algae literally make the air we breathe.