The unique biology of the avocado

HostI was looking at an avocado on my counter this morning and it hit me how weird they are. Most fruits are either sweet or juicy, but this thing is more like a ball of butter in a thick shell. Why did this one plant decide to be so different from everything else in the fruit bowl?

GuestIt really is a bit of a freak of nature. If you look at most fruits, they follow a very simple plan. They pack themselves with sugar and water to get birds or small animals to eat them. The animal gets a quick hit of energy, and the plant gets its seeds moved around. But the avocado went the other way. Instead of sugar, it's packed with oil and fat. It's actually a giant berry, but it acts more like a piece of meat or a nut. It's trying to offer a much bigger meal to a very different kind of eater. It's a high-cost strategy for a plant, but it worked because it was looking for a specific kind of partner.

HostBut why go through all that work? Making fat takes way more energy for a plant than just making some sugar water.

GuestYou're right, it's a massive cost. To understand why the plant does it, you have to look back about ten thousand years. Back then, the Americas were full of giant monsters. We're talking about ground sloths the size of a house and huge, hairy elephants. These beasts had massive bellies and needed a lot of heavy fuel to stay alive. A tiny sweet berry would be nothing to them. But a fat, oily avocado was like a high-power energy bar. They would gulp the whole thing down, pit and all. That giant pit was actually a good thing back then because it was tough enough to survive the trip through a massive gut and come out the other side in a pile of droppings, ready to grow in a new spot.

HostBut those giant animals have been gone for a long time. If the avocado was built for them, how's it still here? If there's nothing big enough to swallow that huge pit anymore, the tree is just dropping seeds at its own feet. That seems like a quick way for a plant to die out.

GuestYou hit on the big mystery. For a long time, the avocado was what some people call a ghost. It was a plant that lost its best friend. In the wild, they probably should've gone away when the giant sloths did. When those big animals died off, the trees were just dropping their fruit and letting it rot right under the branches. That's a bad way to grow a forest because the baby trees can't get any light under the shade of their parents. The only reason we have them today is that humans stepped in. We found them, liked how they tasted, and started planting them ourselves. We basically took the place of the giant sloths and kept the tree alive. Without us, the avocado would just be a weird footnote in history.

HostThere's another thing that drives me crazy. You buy one at the store and it's hard as a rock for a week, and then it's perfect for about ten minutes before it turns brown and gross. Why is the timing so tight?

GuestIt's because of a very strange rule the tree follows. An avocado will never, ever get soft while it's still hanging on the branch. You can leave a full-grown fruit on the tree for months and it'll stay hard as a rock. It's like the tree is a natural storage locker. The fruit only starts to ripen once it's picked or falls off. This was a clever way to keep the fruit safe from the wrong animals. It meant the avocado wouldn't get soft and rot while it was up high where only small birds or squirrels could reach it. It waited for a heavy fall to the ground to signal that it was time to get creamy and ready for a big animal to find it.

HostSo it's just sitting there on the branch, waiting for a fall that never comes?

GuestPretty much. Once it leaves the tree, the fruit starts to breathe much faster and breaks down its own fats to get soft. It's a one-way trip. Once that process starts, it goes as fast as it can because, in the wild, a soft fruit on the ground is a ticking clock. It has to get eaten before it molders or gets eaten by bugs. That's why your kitchen counter feels like a race against time. The plant is still acting like it's lying on a forest floor, waiting for a giant beast to walk by and pick it up.

HostWe have even changed them to fit our own tastes over time, haven't we? The ones in the store don't look like they could've survived on their own.

GuestWe did change them quite a bit. The wild ones were mostly pit with just a tiny thin layer of green flesh. We bred them to be much bigger and creamier. But even though we changed the outside, the core of the plant is still that ancient survivor. It still uses that big pit to protect the baby tree from the strong stomach juices of a giant animal. The tree is still playing by the rules of a world that ended thousands of years ago. It's still waiting for a giant sloth that will never come back, holding onto a pit meant for a mouth that hasn't existed for ten thousand years.

HostThat green mash on my toast feels a lot more like a link to a lost world now that I know I'm basically doing the job of a three-ton sloth.