Homo sapiens, the human species to which most of us belong, has been around for 350,000 years, and earlier representatives of the Homo genus have been identified in the fossil record of 2.5 million years ago – a long history on Earth.
In these spans of time, humans have populated every continent and exploited every ecosystem. Our numbers now approach 8 billion, and our total human biomass is now eight times as great as the total for all living wild mammals. But lest the human ego leads us to think we’re the most venerable and enduring of all existing organisms, consider these.
In modern habitats, horsetails (Equisetum), clubmoss (Lycopodium), and ferns are botanical Methuselahs. These familiar spore-bearing plants dominated ancient forests over 350 million years ago, preceding flowering plants by over 200 million years.
Less than a century ago, a few primitive-looking Indian Ocean fish were discovered in the waters off southern Africa. It took scientists very little time to match these Coelacanths, relatives of lung fish, to some of the very earliest fossil fish, a lineage of an astonishing 400 million years.
As summer nears, if you venture to an Atlantic sandy beach, you may well encounter the most extraordinary “living fossil” of them all: the misnamed horseshoe crab (Limulus polyphemus). The Atlantic horseshoe (they’re not crabs) is found in temperate coastal waters, from the Gulf of Mexico to New England. Paleontologists have samples of horseshoe fossils of some 450 million years ago that bear a striking resemblance to the “modern” species. They share a protective carapace (“shell”) that covers the entire body, a stiff dagger of a tail, and the same systems for feeding, senses, reproduction, respiration, circulation, and mobility.
Over Earth’s life history, most animal and plant species have been lost forever (e.g., the dinosaurs) in mass extinctions caused by tectonic collisions of continental plates; by huge volcanic explosions; or by asteroid impact disasters. The fossil and geological record has defined five of these events (we’re living in the sixth) that produced traumatic climate change, poisoned air and water, and obliterated entire ecosystems, opening ecological doors for new domination of flowering plants and mammals large and small. But extraordinary, resilient life forms, such as horseshoes, coelacanths, and horsetails survived the mass extinctions.
In the taxonomy of contemporary animals, horseshoes are a distinct category within the phylum of arthropods. All arthropods have jointed pairs of appendages, a segmented body, and an exoskeleton. The major categories of arthropods are insects, arachnids (spiders), and crustaceans (crabs, lobsters, shrimps, barnacles). Horseshoes are closely related to the ancient trilobites that are common in the fossil record but were lost in the Permian mass extinction 250 million years ago. Modern DNA analysis has found that horseshoes’ closest relatives are arachnids.
As a child at Cape Cod, I heard tall tales of wading children’s small feet punctured by “gigantic” horseshoe tails or being bitten by a fierce horseshoe crawling across their toes. But despite their defensive design, these scary-looking critters are harmless. Often found upside down in the sand with flailing feet failing to right them, they welcome our assistance by flipping them over. Examining its underside will introduce you to its ten legs and feet and its book gills. Less obvious are its ten eyes, though two are prominent on the surface of its carapace.
Horseshoes live on the shallow continental shelf where they feed on marine worms and mollusks. In the spring, the big female horseshoes gather on the sandy intertidal zone where they ride the high tide then creep as sluggish as a sloth to the upper beach. Several smaller males, drawn to a female’s chemical invitations (pheromones), clasp on to her to fertilize her eggs in her shallow nest of damp sand. She may lay 4,000 eggs in one day at one site and repeat the effort daily for over three weeks. The eggs hatch in the nest and tiny horseshoes catch the next tide to the ocean where, like other arthropods, they grow by molting their exoskeletons and growing larger ones several times in their early years.
The big reproducing horseshoes up to 18 inches long are at least 8-10 years old and molt only rarely. Instead, their tough carapaces become encrusted mobile homes for small barnacles, worms, oysters, mussels, shrimp, sea stars, sponges, and algae.
Those abundant horseshoe eggs are also a vital part of the coastal ecosystem. Migratory shore birds depend upon this food source on their spring northward journey from South America to breeding in the Arctic. Famously, the beaches of Delaware Bay host many thousands of horseshoes laying eggs and up to a million northbound Red Knots (Calidris canutus) and other shore birds who gorge on these protein-rich eggs along the tide line.
The unrivaled adaptive resilience of horseshoe populations has been threatened by the behavior of that evolutionary upstart: Homo sapiens. Our commercial fisheries have exploited horseshoes for bait; our coastal development has disrupted their breeding grounds; and a remarkable feature of the horseshoes – their blood - has been aggressively harvested for pharmaceutical purposes. Horseshoe blood (which is blue) has an immune system that scientists have found to be useful in detecting dangerous bacterial endotoxins that may be introduced to the human blood stream by vaccines, injected drugs, intravenous solutions, joint replacements, or organ transplants and cause severe infection. Their saving our lives!
In 2014 Elizabeth Kolbert published the visionary book, The Sixth Extinction: an Unnatural History, that addresses our current impacts of the earth’s changing climate, collapsing ecosystems, and diminishing biodiversity. But resilient species will survive, species such as horseshoes, coelacanths, horsetails and clubmosses, as well as cockroaches, rats, and humans.