Ask paleobotanist Andrew Leslie what he does and you’ll never think of pine cones the same way again. It turns out their woody scales harbor an evolutionary history that harks back hundreds of millions of years to when land animals first began to forage for food off the ground – in the treetops.
The evidence that Leslie gathered for his dissertation work at the University of Chicago suggests that the emerging hunger of insects and dinosaurs and later birds and small mammals for conifer seeds may have provided a natural selection pressure that forced the primal trees to change how their seeds were housed. The fossil record shows that while conifers started out providing little protection for their seeds, over time the heavily armored – sometimes even spiky – seed cones of today began to take shape. (Pine cones are just one of many seed cone varieties.)
“Seed cones were much narrower, flimsier structures when they first got started in the carboniferous era about 300 million years ago,” said Leslie, who arrived at Brown this summer as assistant professor of ecology and evolutionary biology. “But at some point over the Mesozoic, the Jurassic in particular, the cones become proportionately much wider. That pattern has continued to the present day. It’s a fundamental change that happened around 180 million years ago.”
That extra seed cone girth developed at about the time when taller dinosaurs, tree-dwelling lizards, the earliest birds, and herbivorous beetles had come into existence and presented a potential novel threat to forest canopies.
“In the Jurassic to the present you might have the same size seeds as before but you have much more tissue around them,” Leslie said. “I interpreted that to mean that the plants are investing much more tissue in surrounding the seeds for protection from something that they didn’t need to protect them from before.”
The evolution toward specialized seed cone structures has been complex. Some Jurassic conifers apparently adapted to the emerging predation with a “strategy” that embraced the onslaught. Their cones became fleshy treats that coould be readily eaten so that seeds would be excreted elsewhere.
But conifers that tried to fortify their reproductive hardware against consumption also had to evolve mechanisms by which their walled-off seeds could still be reached by pollen to accomplish fertilization. Leslie is also interested in how current plants work, so he has looked into seed cone pollination mechanisms, including one rather clever approach. The black pine seed cone, as in many other conifers, seems to have evolved a structure that allows it to deploy a drop of water that can catch pollen drifting by in the wind. Pollen grains hitting the droplet float up into a hole at the top of the structure that leads into a tube to the ovary.
At Brown, where both a wind tunnel and a microfluidics lab are available to ecologists, Leslie has the tools to study how the pollen catching structure works and could have evolved.
More generally, Leslie said, Brown provides a welcoming intellectual home for someone who fundamentally is interested in how evolution explains why plants look and function the way they do. There are several faculty members with closely related interests in botany, morphology, and evolution.
But one won’t always find Leslie on campus. As a paleontologist, he’s prone to set out on expeditions. On one such trip to Mongolia during his postdoctoral studies in the forestry school at Yale, he discovered a new species, Schizolepidopsis canicularis, that may be an early relative of modern pines. It was lying in a lignite coal bed.
Leslie has been finding fossils since he was in elementary school in West Virginia. In the hilly terrain where crews had cut through the rock to make roads, his boyhood finds of fossilized clams (West Virginia was once a wetter place) and trilobites kindled an early interest in geology and in ancient life.
In college at the University of Pennsylvania, while he studied biochemistry and geology, he took a course where he had to describe fossil plants.
“It had been a long time since I did any fossil stuff and I really liked it,” he said. “I thought I was OK at it, so I decided I would go to grad school and be a paleobotanist, rather than a chemist which I actually wasn’t very good at.”
The rest was history — history from long, long ago.