400 Million Years of Ferns

Ferns are all leaf, all the time—no celebrated flower, no seduction of the insect. At the tips of green sprouts, curled fiddleheads unroll while leaflets widen behind them like the wakes behind boats. My store-bought Boston fern, tended for years with no expertise, is a fountain of green, its dazzling fans of leaves arching up, out, and over.

Before ferns, plants were creatures of the water, with no need for roots or stems. But about 450 million years ago, Earth passed through the first of its five mass extinctions (so far). The climate cooled, water froze, glaciers grew, the sea level fell, coastal and ocean habitats disappeared. But new land was exposed, and when the climate warmed again, plants grew there.

The first of these land plants were mosses. Carpets of moss sprang up along lakes and streams. But without deep roots or firm stems, mosses grew no higher than a few inches. Thriving on land requires tubes that can carry liquids up and throughout an organism. Ferns were among the earliest plants with roots to dig deep for water, and with stalks and stems to transport it throughout the plant. Vascular tissue changed everything. Ferns reached the height of trees.

Devonian ferns wikimedia

wikimedia

And ferns grew very efficient leaves. They needed them to capture carbon dioxide whose levels fluctuated widely over the millennia. Whenever the atmosphere’s declining carbon dioxide led to cooling temperatures, the hundreds of leaves on even a small fern could soak up enough to keep it going. Today, healthy ferns withstand cold weather better than most plants.

Ferns have multiplied the world over in part because of how they propagate. They were and are old-fashioned and deliberate about sex. Instead of reproducing through miniaturized plants embodied in seeds, ferns spread through spores, single cells released from the underside of the fronds. When the spores settle on moist soil, they grow into a tiny intermediate plant that will create the male and female components for the start for a new fern. Spore propagation seems an overly complicated, two-step process, but the lightness of the spores lets them disperse on the wind and germinate on distant moist soil.

Some days I stare at a plant whose distinctive features date back nearly 400 million years. No other plant or animal that I see every day goes back almost to the beginning of life on land. A fern’s ancestors invented leaves and roots and the old method of reproducing.  They survived four mass extinctions, as well as the flowering plants that burst on the planet 125 million years ago and came to dominate the plant kingdom. Ferns watched the dinosaurs come and go. Today they regale us with tales of ancient climates and their ingenious survival skills.

 

 

“Your Inner Fish”

Most people accept the idea that we are descended from earlier primates. But descended from fish? That might seem to be a stretch. In Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body (2008), Neil Shubin makes a vivid case. He traces our body parts back to fish and beyond. And he comments passionately on the beauty and unity of life’s development over billions of years.

tiktaalik

Tiktaalik, ready for push-ups–and land
(shubinlab.uchicago.edu

Along the way, Shubin narrates the painstaking detective work of the paleontologist. He weaves the opening chapters around his own years in the barren Arctic as he helped to discover, in 2004, the 375-million-year-old fossil of a missing link between fish and amphibians.

The discovery  of Tiktaalik, Inuit for “large freshwater fish,” was exciting for several reasons:

All fish prior to Tiktaalik have a set of bones that attach the skull to the shoulder, so that every time the animal bends its body, it also bent its head. Tiktaalik is different. The head is completely free of the shoulder. This whole arrangement is shared with amphibians, reptiles, birds, and mammals, including us. The entire shift can be traced to the loss of a few small bones in a fish like Tiktaalik. (p. 26 in the Vintage paperback edition).

Say hello to our neck.

neck

In the middle, Tiktaalik’s new neck.
(avonapbiology.wikispaces.com)

There was another surprise about Tiktaalik. Earlier fish had fins with bones that were the ancestors of our arms and fingers. To these early forearms and fingers Tiktaalik added small bones that were, in fact, the earliest wrists. These crude wrists mean that Tiktaalik, as Shubin puts it, “was capable of doing push-ups….The wrist was able to bend to make the fish’s ‘palm’ lie flat against the ground….Tiktaalik was likely built to navigate the bottom and shallows of streams or ponds, and even to flop around on the mudflats along the banks” (39-40).

So Tiktaalik had fins capable of supporting the body—these were the first limbs for moving on land. Since then, the same structure has appeared in not only the arms, legs, and hands of humans and other mammals but also in the wings and flippers of bats, penguins, birds, and whales. In all these, one bone is attached to the torso (in us, the upper arm bone and the upper leg bone), followed by two bones, followed by little bones (wrists and ankles), followed by smaller fingers and toes.

limb bones

Limb bones from fish to humans
(noreligionblog.wordpress.com)

Shubin’s other chapters trace the development of teeth, heads, noses, eyes, ears, and even bodies themselves, which emerged 3.5 billion years ago. In each chapter at some point, Shubin pauses to explain his appreciation of the patterns he is describing. “Beauty” and “beautiful” are common words in the book. The anatomy of the head “is deeply mesmerizing, in fact, beautiful. One of the joys of science is that, on occasion, we see a pattern that reveals the order in what initially seems chaotic. A jumble becomes part of a simple plan, and you feel you are seeing right through something to find its essence” (82). Echoing Carl Sagan’s thought that “looking at the stars is like looking back in time,” Shubin says the same about the human body. “If you know how to look, our body becomes a time capsule that, when opened, tells of critical moments in the history of our planet and of a distant past in ancient oceans, streams, and forests” (184).

I would guess that many scientists feel the same way about looking into the essences behind the apparent chaos. But not many write about it so well. That’s unfortunate, because I think that if they did, more non-scientists would value what our biological history can tell us about who we are and what our lives mean.