I recently encountered a fruit I’d never seen or heard of before: the finger-sop (Meiogyne cylindrica), native to Australia. The thing about the fruit that instantly impressed me was how much it resembled our native North American pawpaws (Asimina species), with one striking difference – instead of a greenish-yellow skin, the skin of finger-sop is bright red. Both fruits are obscure members of one of the ‘royal families’ of tropical fruit, the Annonaceae, which includes such famous delicacies as cherimoya, guanabana, and rollinia. But unlike those relatively recently evolved, glitzy, glamorous fruit superstars, the humble pawpaw and finger-sop appear to retain some of the ancestral characteristics of this fruit family, characteristics which reach way back in time.
Not only is the external appearance of pawpaw and finger-sop similar (aside from the color), the internal appearance is nearly identical, including the arrangement of seeds in the creamy flesh. And so is the flavor: finger-sop has that same mildly sweet, fruity taste, with a creamy texture and distinctive ‘pawpaw’ sort of flavor. The resemblance between the two is so close that if you blindfolded me and handed me a finger-sop fruit to eat without telling me what it was, I’d probably say, “I know this fruit, it’s one of our native Florida pawpaws.”
Since these two fruits share a common ancestor, the simplest explanation for the similar characteristics is that they both retain those characteristics from a common ancestor. So when did that ancestral fruit tree live, producing its oblong, sweet, pawpaw-like fruits? Scientists studying molecular clock analysis have found an answer: the last time these fruits shared a common ancestor was not a million years ago, not ten million years ago, but a whopping 71 million years ago. This is an ancient, ancient fruit type.
In fact, 71 million years ago is a particularly interesting number, because of another date: the time when the giant asteroid impact killed off the dinosaurs, which was 66 million years ago. This means that if the number produced by the molecular clock analysis is accurate, then for about five million years, trees producing pawpaw-like fruits shared the planet… with dinosaurs.
Did dinosaurs eat those ancestral pawpaws? The reason plants make fruits is to entice animals to eat them, in order to disperse the seeds. Dinosaurs were the dominant land animals at that time, filling ecological niches in size ranges all the way from the familiar mega-giant beasts, all the way down to tiny dinosaurs the size of a chicken. Even if the fruits evolved to appeal to some other animal as their main seed dispersal agent, it seems a virtual certainty that over the long span of five million years, hungry dinosaurs would have at least occasionally gobbled down a few of these pawpaw-like fruits. But since dinosaurs were so overwhelmingly dominant, it’s perhaps even more likely that these fruits co-evolved to entice dinosaurs themselves as their main seed dispersal agent.
So there you go. When you eat a pawpaw, you are partaking of a very long tradition, sharing a fruit experience that’s been enjoyed by 71 million years of creatures, including a very different type of biped, which held these fruits in its scaly hands a very, very long time ago.
One of the big questions I had was: how did these two kinds of fruit get to the opposite sides of the earth, Australia and North America? One possibility is bird dispersal. Some fruits can spread across oceans by birds carrying their seeds in their bellies, and dropping the seeds in faraway lands. But fruits adapted to being eaten by flying birds usually have much smaller seeds and fruits than either pawpaw or fingersop – these fruits appear adapted to be eaten by larger, heavier creatures.
So the ancestral fruit species probably would have had to cross over a land connection. I looked up the history of when the drifting continents were attached to each other to try to see if there was a land connection that would have existed between North America and Australia around 71 million years ago, either all at once, or in a successive series of land connections that would have allowed a plant species to spread between these two land masses. I was hoping to find fairly precise dates for when land connections between continents formed and severed, to really nail down the story of these fruits. But I found that while scientists are fairly confident about what the general pattern of continental drift has been, they have a lot more uncertainty about exact times of when land connections formed and broke apart.
I did find one intriguing thing: there’s at least some evidence that between 75 and 65 million years ago, the continents of North America, South America, Antarctica, and Australia may have been linked in one long chain which would have allowed the spread of species all the way across. That fits our timeline perfectly. If that’s true and if that’s how the ancestors of these fruits spread, it’s interesting that a crucial link in that chain would have been Antarctica, which sat over the South Pole then as it does now, but which was densely forested. The Earth was much warmer in the late Mesozoic than it is now.
Although pawpaw and finger-sop are very similar fruits, they have one striking difference: ripe pawpaws are greenish yellow, but ripe finger-sops are bright red. Is there a way to guess which if either of those color patterns the ancestral fruit may have had?
We can make some speculative guesses, based on which kind of animals the fruits seem to be evolved to attract as their seed dispersal partners.
Pawpaws show all the classic features of a mammal fruit. Since most mammals have poor color vision and can’t see the color red at all, fruits adapted to mammals often ripen to a green or dull yellowish color. Mammals generally have an excellent sense of smell, however, so a common trait of mammal fruits is to announce their ripening to mammals by becoming fragrant. Pawpaws do develop a pleasant fruity fragrance upon ripening, but their color hardly changes at all from a solid green in the unripe stage, to a slightly greenish-yellow when ripe.
Finger-sops, on the other hand, turn from green to bright red when they ripen. Among mammals, only some primates and possibly some marsupials can see the color red. Australia obviously has lots of marsupial mammals. Birds can see the color red very well, and Australia is home to cassowaries, large ground-dwelling birds that eat mostly fruit. Australia was until recently also home to giant tortoises, which also can see the color red very well.
There’s another category of animals that are thought to have had excellent color vision, including the color red: dinosaurs. So if our guess is correct that dinosaurs where the seed dispersal agent that the ancestral pawpaws co-evolved with, then it’s quite possible that those ancestral fruits resembled finger-sops, announcing their ripening to hungry dinosaurs by turning bright red. We may have to update our vision of late Mesozoic forests to include dinosaurs traipsing through forests in search of the flashes of bright red that indicated a tree full of sweet, pawpaw-like fruits for them to eat.