WHAT IS THE ANGIOSPERM GAP?

 More than 90% of the plants on Earth are angiosperms, flowering plants whose seeds are enclosed inside the fruit. And it is everywhere -- but exactly how and when these plants came to be so ubiquitous is one of the most stubborn questions in science. Back in the 1800s, Charles Darwin himself called it an “abominable mystery”. And despite nearly a century and a half of working on it, the origins of flowering plants are still a subject of debate. The basic question is simple: When did flowering plants evolve? Our two best sources of data are the fossil record and DNA evidence.

 And they… provide two totally different answers. See, the oldest definite flowering plants in the fossil record are from the Early Cretaceous Period, around 130 million years ago. But scientists can also use genetic information to calculate a date in what’s called a molecular clock estimate. To do so, they compare genetic diversity among living plants and then use the estimated rates at which those plants’ genomes change to calculate how long it would take them to diverge from a common ancestor. And this technique reproducibly comes up with dates that are much earlier than the Early Cretaceous. One particularly large study from 2019 used DNA from more than 2300 living plant species to estimate an origin in the Late Triassic Period, around 209 million years ago.

 So, that’s over 70 million years of difference between the fossil and DNA estimates -- a gap that basically skips over the entire Jurassic Period! Now it’s not unusual for fossils and DNA to disagree. It’s happened before in estimating the origins of birds and mammals. But over time, those disagreements diminished as our information got better… but that hasn’t happened for flowering plants. So… why not? Who is wrong here? Well, on one hand, it could be a fault in our fossils. The fossil record is notoriously incomplete. The chances of any particular plant being preserved can depend on its size, shape, and ecology, and not all plants fossilize equally. 

And the odds are pretty low that the oldest fossilized angiosperm also happens to be the first-ever angiosperm. So maybe there are a bunch of early angiosperms that we just haven’t found, or were never preserved at all. This is the explanation favored by the authors of that 2019 genetic study. They even proposed a name for this missing history: the Jurassic angiosperm gap. But here’s the thing … some paleontologists are pretty sure there isn’t a gap at all. A separate 2019 study did a major review of the oldest known flowering plant fossils, specifical pollen. Pollen is as great for paleontologists as it is bad for people with seasonal allergies. Those little grains are durable, and they get everywhere.

 The study confirmed that angiosperm pollen goes back to the Early Cretaceous. Now, there are fossil sites even older than that where we can find pollen from other kinds of plants, including close relatives of angiosperms. But those sites don’t have angiosperm pollen. We usually hear that absence of evidence is not evidence of absence, but in this case, the study authors say, the lack of pollen seems to indicate that angiosperms really just weren’t there. And there’s more: when flowering plant pollen does finally appear in the fossil record, it does it in the order we’d expect.

We know from genetic studies which varieties evolved in what order, and that’s the order we see their pollen appear. That really makes it look like the early evolution of angiosperms happened in the Early Cretaceous. It could still be an illusion of fossil preservation, but it’s also possible that our genetic estimates could be wrong. See, despite the name, a molecular “clock” isn’t a date written in a piece of DNA. It’s a complex mathematical model, and what that model puts out depends on the data you put in. That starting data can include relationships between plants, the rates those plants evolve, and the age of the fossils of angiosperms and their relatives.

 If we’re wrong about any of that, it’s going to affect the date a molecular clock analysis spits out. In fact, studies testing these models have found that wrong inputs often lead to too-early dates. If that’s happening here, then the supposed “angiosperm gap” might be a result of our own misunderstandings of plants. But so far, genetic studies consistently place angiosperm origins well before the earliest fossils.

 It might be that the true answer is somewhere in between. Molecular clock estimates usually aren't a single date, but a range of possible answers and many of these estimated ranges have younger ends in the Middle or Late Jurassic Period. And even the fossil pollen study noted that, even with the fossil evidence at hand, slightly earlier angiosperms could have existed in the later Jurassic before becoming common enough to show up in the fossil record and evolve the forms we recognize.

 Ideally, fossils and DNA will work together to arrive at the right estimate. More fossils of flowering plants and their relatives will help us better understand the timing of key evolutionary steps. And more careful consideration of the data on plant evolution will help us narrow down those molecular clock estimates. Resolving this puzzle isn’t just important for knowing where our favorite flowers come from. It’ll also help us understand past environmental changes and the history of animals like insects who evolved alongside flowering plants. In the end, there will be a right answer. But there’s bound to be some more data tug-of-war before we find it.