400 Million Years of Ferns

Ferns are all leaf, all the time—no fanfare for a celebrated flower, no seduction of the insect. At the tips of slender sprouts, curled fiddleheads unfurl while leaflets widen behind the unrolling tip, like a wake spreading behind a boat. My store-bought Boston fern, tended for years with no expertise, is a fountain of green, bearing dazzling fans of leaves that arch up, out, and over.

Before ferns, all plants lived entirely in the water, where they did not need roots or stems. But about 450 million years ago, Earth endured the first of its five mass extinctions to date. 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 stalks and stems to transport it throughout the plant. Vascular tissue changed everything. Ferns reached the height of trees.

Devonian ferns wikimedia


And ferns became expert at growing efficient leaves. They needed to do so to capture carbon dioxide whose levels fluctuated over the millennia. Declines in the atmosphere’s carbon dioxide led to cooling temperatures. But the hundreds of leaves on even a small fern were ready to soak up enough carbon dioxide to keep the fern going. Today, healthy ferns withstand cold weather better than most plants.

Ferns have multiplied the world over in part because of how they propagate. In this respect, they were and are old fashioned. 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 a tiny intermediate plant that will create the male and female components for the start for a new fern. Spore propagation seems a complicated two-step process, but the lightness of the single-celled spores lets them disperse on the wind and germinate on distant, moist soil.

Some days I gawk 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 start of life on land. A fern’s 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 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.



Survivors and the Terminator

The story of evolution recounts the ways that plants and animals have changed over time as small bodily variations have improved their odds for survival. But what about those species that fitted so successfully early on into stable environments that they have altered little or not at all? Are any such ancient species (besides the microbes) still around today?

Yes. In Survivors: The Animals and Plants that Time Has Left Behind (2011), Richard Fortey tracks down present-day plants and creatures that look much like their fossilized ancestors. Oddly, their stories of sameness impress me with as strong a sense of evolution’s power as stories of modification; natural selection goes with what works, whether it’s new or old.

Horseshoe crabs, after half a billion years, still crowd the beach. (delaware-surf-fishing.com)

Horseshoe crabs, after half a billion years, still crowd the beach.

In the opening chapter, Fortey is on a beach in Delaware on the night of the annual horseshoe crab orgy, when thousands of male horseshoe crabs come ashore to clamber on top of the females. Horseshoe crabs have  been scrambling up beaches for 500 million years.

In the darkness along Delaware Bay the scratching percussion of the crabs provides an unmusical accompaniment on an imaginary journey backwards in time: to an era well before mammals and flowering plants; a time before the acme of giant reptiles, long before Tyrannosaurus; backwards again through an extinction event 250,000,000 years ago that wiped nine-tenths of life from the earth; and then back still further, before a time of lush coal forests to a stage in the earth’s history when the land was stark and life was cradled in the sea.

firmoss (newfs.s3.amazonaws.com)

Four hundred mya, plants like this Norwegian Huperzia moved from the water to land and were among the first to acquire the crucial, stiffened water-conducting tubes–today we know them as stalks–that enabled them to stand upright and compete for light.

In the other chapters, Fortey describes the velvet worm in New Zealand, the lovely Norwegian fir moss, the not-so-lovely lungfish. The photo captions here mention their keys to long-running adaptive success.

It helps that their environments have changed very little. “Survival is about endurance of habitat.” One habitat that endures is the tidal flat, where the shallow sea meets the muddy shore. Organisms here can burrow, find oxygen in the water and air, find food through filtering or scavenging. Here are horseshoe crabs, snails, small fish, many plants. “This habitat does seem like a good place to be for an organism with conservative tendencies. If its own place survives, then so will the beast. It is the right place to weather mass extinctions that affect many other environments more severely.…Stick-in-the-muds last longest.”

As for the durable species themselves, a few characteristics recur. “Many…seem to have long life spans.” Horseshoe crabs take a decade to mature. And some enduring animals invest more resources in fewer progeny by producing “relatively large eggs or few offspring.” But there are no fool-proof formulae. “[T]he luck for old timers will eventually run out. It always does.”

lungfish (media-web.britannica.com)

In Australia, Fortey finds the lungfish that dates back 400 mya, “the great survivor among the vertebrates, the animals with backbones.” With lungs, these sea creatures, in transitioning to the land, can gulp air whenever the river runs low on oxygen.

What about humans? When will our time run out? Fortey doesn’t make guesses, but he has a hard label for us.

[Early man] may have hunted edible mammals and birds to the point of obliteration even in his earliest days; he was the first species that deserves to be called the Terminator….The extinction event that is happening right now is the first one in history that is the responsibility of a single species. There’s no meteorite this time, no exceptional volcanic eruptions, no ‘snowball earth,’ just us, prospering at the expense of other species.

He is confident about the survival of only one kind of organism. “I am not worried about the survival of bacteria. They will be there to rot down the last bodies of the last humans, and then the wheel of life will have turned full circle.” So ends the book.