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.


Is DNA Alive?

No, it’s not alive…mostly. The only sense in which a DNA molecule is a living thing is that it makes copies of itself, although it can’t even do that on its own. Otherwise, DNA fails all the tests: it doesn’t process any kind of fuel in order to maintain its state, it doesn’t grow and develop, so it has no energized activity that starts or ends—in other words, it’s not born and it doesn’t die.

Somewhere along the line in reading general science I picked up the impression, even though I think I knew differently, that DNA strands are alive. They are such vital keys to living organisms, and I’d read so many descriptions of what DNA does and of “selfish” genes, that although I knew they were blueprints of a sort, they came to seem like living blueprints.

DNA and seed (kew.org)

DNA and seed

One image that took shape in the back of my mind was that DNA was a kind of seed, and seeds, I thought, are alive. But no, seeds are not fully alive either. They are not active and, until they germinate, they don’t change or develop. (Another familiar item that may seem alive but that doesn’t meet all the criteria are viruses. Viruses are bundles of DNA that become active only when they are inside a cell, at which point they take over the cell and give us the flu.)

It shouldn’t be surprising that some familiar biological components do not, by themselves, meet all the criteria for the complex condition we call “being alive.” Still, surprised I was about DNA. Perhaps because we humans are so fully aware that we are alive, it is easy to think that there must be a fully living seed or even a soul at the root. It is almost more than we can imagine that the liveliness we feel is the product of a complexity of non-living parts. It’s an astounding thing.

My Million-Year-Old Back Yard

I like knowing the age of living things—not the age of the individual organism but of the lineage, of how long a plant or animal has been different from other lineages. The dates give me a glimpse of a Past that, like a god, generates and then consumes everything.

So here are the ages, youngest first, of the plants and animals in my suburban yard. Dates are approximate by millions or tens of millions of years!

The youngest creature in the yard is our dog, an animal that separated from its wolf-like ancestors about 40,000 years ago.

Next is me and my wife. Our species, Homo sapiens, separated from our Homo ancestors about 200,000 years ago. Before that, our genus, Homo, split off from the genus that chimps belong to about 7 million years ago. All our ancestors in our genus have died off and we are the only member left, a strange isolation. We are probably the only species in the yard in that situation.

The youngest plant is the grass, appearing about 40 million years ago among plants that adapted to a warming climate.

Back yardThe first squirrel fossil dates from 36 mya. Squirrels are part of a huge group of rodents with big, continually growing teeth. The chipmunks are in the same category.

An oak tree dominates the yard. Although trees in general have been around for much longer, the oak was part of the spread of flowering plants, at very roughly 70 million years.

There’s a holly tree. The several hundred species of holly emerged about 80 million years ago.

Other flowering plants and trees come next. Their ancestors began diverging from flowerless plants around 240 mya, they were blooming 160 mya, and they became widespread and then dominant among plants during the 100 million years after that.

Insects originated about 600 million years ago, but the modern insects in the yard—flies, butterflies, wasps, bees, ants— co-evolved along with the flowering plants from 146 to 66 mya.

The birds are thought to have evolved from certain dinosaurs that carried feathers for warmth, about 180 million years ago.

The pine trees and cedars around the house are among the conifers that date back 300 million years when early trees began to live away from the water. Conifers reproduce through exposed seeds (on pine cones) and pollen. Protected seeds, enclosed in nuts and fruits, came later.

Two other back-yard inhabitants go back as far as the conifers: Ferns, not so different 350 million years ago, with their tiny, single-cell spores, another predecessor of the modern seed. And spiders, spinning their silk about 300 million years ago.

I’m realizing, as I finish hunting through Wikipedia for these dates, that my intentions have become a little muddled. The “birth” of these species was more process than event, a long interweaving with their early kin. The age of a rose depends on whether you look at it as a rose or a seed-bearing plant or a land plant.

Still, I savor the majestic history here, the story of life.