Wonders of the Oceans
Creatures
December 2025
December 2025
The oceans.
They cover 70% of the Earth's surface and are home to more life than anywhere else. They are widely considered the place where life first evolved, and they play a major role in regulating climate and driving weather.
We look at the origins of life and a few of the fascinating creatures that dwell in the Seven Seas* - the living fossils and the alien intelligences of the deep.
Hydrothermal Vents and the Beginnings of Life
Life is widely believed to have first evolved in the ocean, likely near deep-sea hydrothermal vents, which provided the necessary chemical energy and minerals for the first life forms - microbes - to emerge. The process occurred some 3.7 billion years ago and appears to be something like this:
Hydrothermal vents release geothermally heated, mineral-rich water, providing a consistent energy source for chemical reactions. The temperatures in the immediate vicinity of these vents can be as high as 750F to drive these deep sea reactions and allow inorganic molecules - i.e., those not containing carbon atoms -to be converted into organic compounds like amino acids. The extreme heat mixes with cold seawater, and temperatures drop to the 30-4oF range just a few meters away. Unique ecosystems can develop here, with organisms like tube worms, clams, and shrimp relying on chemosynthesis rather than sunlight.
Once life emerges, the vents could provide a sheltered environment for these early organisms. Today. hydrothermal vents re studied as possible analogs for early Earth conditions and even for potential extraterrestrial life (e.g., on Europa or Enceladus**).
Sidebar-1: NOAA image of a hydrothermal vent
Sidebar-2: Mineral-laden water emerging from a hydrothermal vent on the Niua underwater volcano in the Lau Basin, southwest Pacific Ocean. The microorganisms that live near such plumes have led many scientists to suggest them as the birthplaces of Earth’s first life forms. Copyright: U.S. Geological Survey/Schmidt Ocean Institute
From its humble beginnings as single-celled micro-organisms in the oceans, life evolved slowly and then flourished in the oceans over the next three billion years. [TIMELINE Sidebar]
Three of the most significant events in this progression were the Great Oxygenation Event, the Cambrian Explosion, and the evolution of lobe-finned fish.
The Great Oxygenation Event was triggered when photosynthesis reached a critical level, transformed Earth's atmosphere and oceans and allowed more complex life to evolve.
Science News Today describes the Cambrian seas as full of “soft bodied organisms, armor plated predators, and primitive ancestors of every modern animal”.
Lobe-finned fish had strong, limb-like fins, primitive lungs and the ability to gulp air.
Finally, some 350 million years ago - nearly 3.5 billion years after life first arose, amphibians made landfall.
---Science News Today continues the story of our evolution in "From Fish to Human: Tracing the Incredible Journey of Evolution."---
The saga of sea creatures did not end with amphibians making landfall. Several classes of animals attempted to return to the sea in the ensuing years.
300 to 250 million years ago, marine reptiles went back to the sea. Ichthyosaurs, plesiosaurs, and other species evolved around the same time as the dinosaurs. These marine reptiles died out long before the extinction event that ended the age of the dinosaurs, and today's marine reptiles such as sea turtles, marine iguanas, and saltwater crocodiles are not descended from those ancient species.
Mammals had a better go of it. Whales, dolphins, and porpoises (the cetaceans) evolved from land‑dwelling, hoofed mammals. Their return to the ocean began during the early Eocene, around 50 million years ago. Cetaceans are highly intelligent. Some - dolphins and several species of whales - are also highly communal, placing them among the most social and culturally complex animals on Earth.
TIMELINE
3.7 billion years ago - single-celled microbes
3.0 billion years ago - cyanobacteria develop photosynthesis - the the process used to convert sunlight, water, and carbon dioxide into their own food (sugars) and oxygen.
2.4 billion years ago - photosynthesis reaches a critical level and triggers the Great Oxygenation Event, transforming Earth's atmospheres and oceans and allowing more complex life to evolve
2.0 billion years ago - complex cells with nuclei evolve, giving rise to algae, protozoa and eventually other multi-celled organisms.
1.0 - 0.6 billion years ago - simple multicellular organisms appear - sponges and other soft-bodied organisms
540 million years ago (mya)- The Cambrian Explosion. A dramatic burst of evolution fills the oceans with Arthropods, Mollusks, and the early ancestors of vertebrates.
520-450 mya - Early fish and the first vertebrates - these fish dominate the oceans and set the stage for land vertebrates.
420-380 mya - Lobe-finned fish evolve.
375-360 - the first tetrapods (four-limbed vertebrates)... fish begin walking
360-330 mya - the first amphibians - they live in fresh water swamps and must return to the ocean to reproduce
Living Fossils: horseshoe crabs and coelacanths
Some species that evolved hundreds of millions of years ago are still with us today. Often referred to as "living fossils", they give us a glimpse of what our ocean ancestors looked like in the aftermath of the Cambrian Explosion.
One such living fossil is familiar to those of us living near the Atlantic coast. The horseshoe crab looks like it belongs in the age of ancient marine reptiles, but its look-alike ancestor, which evolved 450 million years ago, actually predates them by 200 million years. They live about 20 years on average, and some females can grow to more than 30 inches long.
Among the interesting aspects of its biology:
The head is the largest part of the horseshoe crab's body and contains much of the nervous and biological organs. The head has the brain, heart, mouth, nervous system, and glands—all protected by a large plate. The head also protects the largest set of eyes.
Horseshoe crabs have nine eyes scattered throughout the body and several other light receptors near the tail. The two largest eyes are compound and useful for finding mates. The other eyes and light receptors are useful for determining movement and changes in moonlight.
Horseshoe crabs have 10 legs and a hard exoskeleton, which allows them to scuttle across the ocean floor. They are bottom‑feeding predators and scavengers, and their diet includes worms and small mollusks such as clams and mussels.
They can survive for four days out of the water as long as they keep their gills wet. You can learn more about how to help a stranded, overturned horseshoe crab survive at this link.
Another such living fossil is the coelacanth. These lobe-finned fish evolved with the rest of their brethren about 400 million years ago. Until 1938, when a coelacanth (pronounced SEE-la-kanth) was found off Africa's coast, scientists had believed the fish went extinct 65 million years ago with a related lineage of prehistoric fishes.
One documented specimen of these large, slow‑moving deep‑sea predators with thick muscular bodies and heavy scales measured 1.452 meters (4.76 feet) long and weighed 53.75 kg (118 lbs). Not only are they among the oldest species, but they are also among the longest-lived - with average lifespans ranging from 60 to 100 years.
Among the other interesting aspects of coelacanth's biology:
Coelacanths are one of the last surviving members of the lobe‑finned fish (Sarcopterygii), with fleshy, limb‑like fins supported by internal bones. This fin structure is similar to the early anatomy that eventually gave rise to four-limbed animals (tetrapods).
Coelacanths move their paired fins in an alternating, diagonal gait, similar to how tetrapods walk on land. This unusual movement pattern is part of why they’re so important in studying vertebrate evolution.
Modern coelacanths live in deep waters off Africa and Indonesia, often between 150–700 meters. Their bodies are adapted for low‑light, high‑pressure environments, including a slow metabolism and specialized sensory systems (e.g., a special detector in its snout that lets it detect electrical signals. This ability helps coelacanths survive in a world where light is almost nonexistent, prey hides in crevices, and slow, energy‑efficient hunting is essential. They don’t chase things. They sense, drift, and strike — a perfect strategy for a deep‑sea animal with a slow metabolism and a long lifespan.
Horseshoe Crab - Picture Credit National Geographic
Coelacanth - Picture credit amazinglife.bio
Alien Intelligences: cephalopods
The earliest cephalopods evolved about 500 million years ago , with modern species such as the octopus, squid and cuttlefish appearing 300 million years ago. In his 2016 book Other Minds, philosopher of science and diver Peter Godfrey-Smith examines the remarkable intelligence of the cephalopods. Discussing octopus behavior, he points to their problem-solving skills, curiosity, ability to navigate mazes and play behavior and signs of memory. In addition they show signs of having "memory" - octopuses don’t just learn — they retain information, sometimes for long periods, and use it to guide future behavior. Their memory systems are as sophisticated as those of many vertebrates.
One of Godfrey‑Smith’s most striking points is that octopus arms can act semi‑independently, solving small problems on their own. This decentralized intelligence is unlike anything in vertebrates, which why he sees octopuses as a “second experiment” in the evolution of complex minds. The octopus, he says, is "probably the closest we will come to meeting an alien intelligence."
The common octopus, one of the best known species typically reaches about 3 feet across when spread out, though other species can be much smaller or larger. While coelacanths can live to a ripe old age of 60 or even 100 years, the octopus's lifespan is short - ranging from just 1 to 3 years.
Below left is a post that captures a diver and octopus communicating in the wild. Below right is a video of a laboratory test of the octopus's intelligence.
Another cephalopod, the cuttlefish, is known both for its ability to camouflage to avoid predators and capture prey and for its amazing mating color-change display. [YouTube video below left] Like the octopus, they are highly intelligent but they possess a different set of skills. One of the cuttlefish's more recently discovered skills is its ability to wait for better food. This self-control is one linked to the higher intelligence of primates . Depending on the species, cuttlefish can be 1.7 to 20 inches long. Like the octopus, cuttlefish have short lifespans - 1-2 years.
For a deep dive into their biology, check out this YouTube video: The Insane Biology of the Cuttlefish. For a shorter look at these colorful creatures, see the video below right.
Notes:
*The term dates back to at least 2300 BC. Over time, different cultures used it to describe the waters they knew best, the number seven carrying symbolic weight across cultures. Although the term today is more poetic than literal, Seven Seas can refer to the seven major oceanic divisions: the Arctic, North Atlantic, South Atlantic, North Pacific, South Pacific, Indian, and Southern Oceans.
**Enceladus, a moon orbiting Saturn, and Europa, a moon orbiting Jupiter, are considered the strongest candidates in our solar system for finding extraterrestrial life (microbial life as on early Earth):
1. Water is the single most important ingredient for life as we know it. Both moons have liquid water oceans lying beneath an icy crust.
2. Life needs energy. Both moons have tidal heating and possibly hydrothermal vents.
3. Life needs the right chemistry. Scientists believe that on Europa, the interaction of the subsurface ocean with the rocky seafloor could produce the minerals, salts, and organic molecules necessary for microbial life. The space research Cassini mission detected fresh, complex organic compounds — including nitrogen and oxygen bearing molecules — in the plumes spewing from the moon's surface. These are the chemical building blocks of life.
***Horseshoe crabs are genetically more similar to scorpions and spiders than they are to today's crabs. The name “horseshoe crab” originates from the rounded shape of the head, because just like the shoe on a horse’s foot, the head is round and U-shaped
Other Sources: https://news.uchicago.edu/explainer/origin-life-earth-explained, https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Horseshoe-Crab, Wikipedia, Coelacanths Can Live Past 100, Don't Show Age? | National Geographic, Other Minds: The Octopus, the Sea, and the Deep Origins of Consciousness - Book Summary :