Life on Earth began approximately four billion years ago, shortly after the formation of the planet. It started, as most things do, in the simplest way possible: single-celled microorganisms like bacteria and archaea, lacking a cell nucleus and organelles.
It took another two billion years for more complex eukaryotic life to emerge, with most evidence suggesting it originated in the ocean.
About 500 million years ago, Earth experienced what scientists refer to as the “Cambrian explosion”–a period of rapid and dramatic diversification of life forms, marking the emergence of most of the major animal phyla that still exist today. For example, arthropods, mollusks, and chordates appeared during this period. Still, this rapid expansion of life on Earth was confined to the oceans. So, whenever scientists speak of the oldest living animal lineages, they are referring to marine animals such as the nautilus, coelacanth, or horseshoe crab.
How and why life transitioned from sea to land is still largely a mystery. We know that terrestrial life began to take root during the late Silurian and early Devonian periods, around 400 million years ago. We also know, through fossils, some of the earliest species to do it. For instance, the famous Tiktaalik, a fossil discovered in arctic Canada in 2004, lived around 375 million years ago and represents a crucial transitional form between fish and the first land-dwelling vertebrates. It had a fish-like body with robust, limb-like pectoral fins, and features such as a movable neck and both gills and lungs, indicating adaptations for both aquatic and terrestrial environments.
Tiktaalik, however, did not last long. Scientists estimate that it either died out or evolved into other life forms by the end of the Devonian period (360 million years ago).
Other lineages of early land animals, however, found a way to live on–not only through the Devonian period but all the way to present-day. Here are two land animals that have survived over 400 million years–not to mention through four mass extinctions–in a relatively unchanged form as their earliest ancestors.
1. Scorpions
The oldest evidence of scorpions on Earth comes from the Silurian period, around 430 million years ago. Fossils of early scorpions have been found in sedimentary rocks from this time, such as Dolichophonus loudonensis which is the earliest known scorpion coming from fossil deposits in Scotland. While there is still debate as to whether these early scorpions were terrestrial or aquatic, recent research indicates that these arthropods were among the first to venture onto land.
Scorpions are considered living fossils due to their long evolutionary history and the relatively unchanged nature of their basic body plan over hundreds of millions of years. For instance, modern scorpions retain many of the same features as their ancient ancestors, such as their segmented bodies, pincers (chelae), and the distinctive tail with a venomous stinger.
2. Cockroaches
Cockroaches are another animal worthy of the title “living fossil.” Their ancestors, known as “blattopterans” or “roachoids,” appeared around 350 million years ago—long before the age of dinosaurs. Although not true modern cockroaches, roachoids were the common ancestors of cockroaches, mantises, and termites, and bore a strong resemblance to today’s cockroaches.
While there have been many adaptations to different environments and ecological niches, the core features of cockroaches have remained consistent, including their flattened bodies, long antennae, and hard exoskeleton.
The modern American cockroach (Periplaneta americana), shown above, closely resembles its ancient ancestors. Fossils of ancient roachoids from the Carboniferous period show many similarities to the American cockroach, including their general body plan and wing structure.
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