400 Million Years of Ferns

Ferns are all leaf, all the time—no slow preparation for the momentous flower, no seduction of the insect. At the tips of young fronds, fiddleheads unfurl, fronds lengthen, leaflets appear and widen behind the unfurling tip like the widening wake behind a boat. My store-bought Boston fern, tended for years with no expertise, bears dazzling fronds that arch up, out, and over—a fountain of green.

Ferns were among the earliest plants with roots to dig deep for water and vascular stalks and stems to transport it throughout the plant. They were the first to grow arrays of hundreds of leaves. This was almost 400 million years ago. Earlier plants had lived entirely in the water, with no need for deep roots. On land, before the ferns, the first mossy plants, lacking roots or stems, could grow no higher than a few inches. Vascular tissue—tubes that conduct fluids, minerals, and gasses—changed everything. Ferns reached the height of trees. And today vascular systems circulate the red blood through us all.

Ferns emerged late in the Devonian Period, which lasted from 419 to 359 million years ago. About 30 million years prior to the Devonian, the first of Earth’s five mass extinctions took place. The climate cooled, water froze, glaciers grew, the sea level fell, coastal and ocean habitats disappeared. As the earth warmed again, carpets of moss sprang up along lakes and streams and bony, heavy-jawed fish swam in the seas. Then ferns and other plants became the first forests.

Devonian ferns wikimedia


Toward the end of the Devonian, the second mass extinction took place. It came in three pulses. Ocean species, including the heavy-jawed fish, disappeared. But land plants, including the ferns, were mostly spared. In fact, one theory  (bbc.com/earth/story) holds that the roots of the ferns and other plants broke through rocks and released nutrients into the lakes, rivers, and oceans. The nutrients fed huge blooms of algae which later died and decayed, taking up the oxygen that had kept the fish alive. Meanwhile, again, the ferns thrived.

Ferns proliferated in part because of how they propagated. Here they were—and are—old fashioned. Instead of reproducing via 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 a tiny intermediate plant that will provide the fertile start for the new fern. Spore propagation seems a complicated two-step process, but the lightness of the single-celled spores let them disperse on the wind and germinate on distant, moist soil.

Some days I gawk at a plant whose distinctive family features date back 400 million years. No other plant or animal that I see every day goes back almost to the start of life on land. The ancestors invented leaves and roots but kept 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 dominated the plant kingdom. Ferns watched the dinosaurs come and go. Today they regale us with tales of ancient climates and their durable adaptations.


Note: Among the sources on ferns, Don Lubin’s “Introduction to Ferns” is especially readable and informative for the general reader.


“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, ready for push-ups–and land

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.


In the middle, Tiktaalik’s new neck.

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

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.