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.

Water is heavy. When we move enough of it, it can change the tilt of Earth’s axis.

It’s that axial tilt that creates our seasons. And the tilt is always changing, due to changes in Earth’s gravity.

Natural phenomena like earthquakes and volcanoes can redistribute the weight of continents, affecting Earth’s gravity. But so can the movement of water.

Researchers recognized that water melting out of glaciers and ice sheets into the ocean had redistributed the weight of that water from the colder latitudes to the equator, where seawater accumulates in a bulge around Earth.

When they calculated the resulting gravitational changes, their numbers didn’t add up. Until they also factored in the weight of the groundwater humans had extracted.

We use groundwater mainly for irrigation and in city water systems, which discharge into rivers that eventually lead to the sea. This moves groundwater from where it’s concentrated underground to soils and oceans.

Across the globe, humans have pulled more than two trillion tons of water from aquifers over two decades. And, the scientists realized, weight redistribution from groundwater alone had changed the tilt of Earth’s axis, causing the North Pole to move three more feet!

They plan to use this new understanding to look for other connections between Earth’s water and gravity, and how droughts might also have altered Earth’s axial tilt.

Anyone who’s been in an airplane has heard that constant roar of flight. That’s partly the engines—but the noise comes more from air rushing over the wings.

Anyone who’s seen an owl fly… hasn’t heard anything. Because unlike an airplane, owls fly almost silently.

How do they do this? Aeronautical engineers have examined owl wings and found several features working together:

The leading edge of the wing, called the comb, is finely serrated, to break up the air flowing over it.

The middle of the wing is covered in soft feathers called velvet, which dampen the high pitch of rushing air that would be audible to rodents and other prey.

The trailing edge has a wispy fringe which further silences the moving air.

In a technique called biomimicry, engineers design products inspired by successful plant and animal features. We’ve covered several in prior EarthDate episodes.

Now they’re looking to adapt the owl’s silent flight to our technology.

New designs have shown that adding a sound-absorbing middle section and a porous, flexible trailing edge to an airplane wing can cut its noise by 25%.

Adding plastic finlets to the leading edge of a wind turbine blade can cut its noise in half!

Owls have other remarkable features: Their eyes are up to 100 times better than ours. And their ears are the most sensitive of any animal ever tested.

What else might engineers learn from this wise old bird?

Like the legendary unicorn it resembles, the narwhal is an enigma.

It lives only in Arctic waters, in a frozen seascape far from civilization, making it hard to study and therefore not well known to science.

For instance, we don’t know what it does with its most distinctive feature.

We do know that its 6- to 10-foot tusk is actually a single tooth, which grows through its lip and continues growing throughout the narwhal’s life.

And that narwhals living farther north have thicker tusks than their southerly cousins.

Some scientists have thought the tusk is used by males to show dominance, or to joust with other males. But some females have tusks too.

Some theorize it’s a sensory organ allowing the whale to test temperature or salt levels in water. But if so, all narwhals should have one.

Narwhals don’t use their tusk for spearing or scaring prey, though their prey species are now changing. We’re not sure why.

Narwhals are known to be some of the deepest-diving cetaceans, going down more than a mile. But we don’t know what they do down there.

Scientists saw a group of narwhals swimming erratically off the coast of Greenland, so they put a tracker on one male and followed it for three months. But its path was so seemingly random they had to use chaos theory to try to make sense of it.

Perhaps the narwhal is just unknowable today, making the unicorn of the sea all the more intriguing.

Antarctica is a continent of extremes and superlatives.

It’s the coldest continent, with Earth’s lowest recorded temperature of 130 below zero Fahrenheit.

It’s the driest continent, with just 6 inches of precipitation a year.

It’s the windiest continent, with gusts up to 200 miles an hour.

Surprisingly, it’s the highest continent, with an average altitude of 8,200 feet, largely due to ice sheets up to 15,000 feet thick.

It also has an extreme range of geologic age, with rocks nearly 4 billion years old on one end, and young volcanoes forming on the other.

Hundreds of millions of years ago, Antarctica sat within the supercontinent of Pangea. When Pangea began to break up, more than 80 million years ago, Antarctica ended up alone at the southern end of the Earth.

When the climate cooled 34 million years ago, Antarctica began to ice over. Over millions more years, as glaciers advanced and retreated on Earth dozens of times, ice built steadily on Antarctica, such that 98% of its land is now covered in ice, hiding what lies below.

As a result, we know less about Antarctica’s geology than that of the Moon or Mars.

But with advanced satellite sensing, scientists are now able to look below the ice, to map a continent of massive bluffs, erosion zones and ancient river drainages that still shape the flow of ice today.

After the dinosaurs, giant mammals ruled Earth for millions of years—then suddenly, most of them disappeared. What happened?

When the Chicxulub asteroid struck, and plunged Earth into darkness and cold, large dinosaurs became extinct.

Early mammals—small, warm-blooded and covered in fur—were able to survive. And, over a few million years, they grew in size to occupy the environmental niches the dinosaurs had vacated.

There were bears the size of rhinos, and rhinos the size of elephants. There were 400-pound beavers and 10-foot-tall ground sloths. They existed from 50 million years ago until around 50,000 years ago. Then, rather suddenly, most went extinct.

There are two hypotheses. As Earth’s climate warmed, some areas became too wet and boggy to support large animals.

Other areas dried out and lost their vegetation. Without plants to eat, herbivores starved. Without herbivores to eat, carnivores starved.

Who didn’t starve? Us. Humans.

The other related hypothesis is that humans, as we migrated around the globe, found large mammals to be an excellent food source.

Hunting pressure on these slow-reproducing animals, especially in Australia and North America, may have caused their populations to crash.

In Africa, though, where humans originated, we’d reached more of an equilibrium with megafauna, and 80% of large mammals still survive.