The question of what fish humans evolved from has fascinated scientists and philosophers alike for centuries. Recent advances in genetics and the fossil record have brought us closer than ever to uncovering our piscine past.
If you’re short on time, here’s a quick answer to your question: modern humans evolved from lobe-finned fish that developed rudimentary digits and the ability to walk on land around 375 million years ago.
In this comprehensive article, we’ll dive deep into the evolutionary history that connects us to fish, examining pivotal transition species like Tiktaalik and key anatomical changes that enabled vertebrates to leave the water and colonize the land.
Overview of Vertebrate Evolution
Origin of Vertebrates
The evolutionary origin of vertebrates is an amazing story spanning over 500 million years. The first vertebrates evolved from primitive chordates during the Cambrian explosion some 530 million years ago. These early vertebrates were jawless fish with skulls and backbones.
The two main groups of jawless fish were the armor-plated ostracoderms and the eel-like conodonts.
About 420 million years ago, the heavily armored ostracoderms evolved into the first jawed fish, called placoderms. Placoderms had jaws made of bone and teeth-like structures called denticles. This was a huge evolutionary leap, allowing vertebrates to become active predators instead of just filter feeders.
Placoderms eventually went extinct, but gave rise to the two major groups of jawed vertebrates that exist today: cartilaginous fish and bony fish.
Transition to Land Vertebrates
Around 380 million years ago, some bony fish started exploring land and evolved the ability to breathe air. These were the first tetrapods, meaning four-limbed vertebrates. The Late Devonian period is sometimes called the “Age of Fishes” because of the remarkable diversity of fish species that existed, as well as the transition to land vertebrates.
Early tetrapods like Tiktaalik and Acanthostega had leg-like fins and lungs, but still spent time in the water. Later tetrapods evolved true legs and feet and became fully terrestrial. These early land vertebrates laid eggs in the water and had tails and webbed feet for swimming.
Around 350 million years ago, tetrapods evolved the amniotic egg that could be laid on land. This was a crucial adaptation that allowed vertebrates to permanently move onto land.
The amniotic egg allowed embryo development away from water. It contained membranes called amnions that provided a protective sac of fluid for the embryo. This facilitated further adaptations like stronger water-retaining eggshells and elimination of gills and tails.
The first truly terrestrial tetrapods evolved from these amniote ancestors around 300 million years ago, giving rise to the ancestors of reptiles, birds, and mammals.
Key Transitional Species
Tiktaalik
Tiktaalik is one of the most important transitional fossils linking fish to tetrapods. This 375 million year old fossil has a mixture of fish and tetrapod characteristics. Tiktaalik had fins with arm-like bones as well as a primitive neck, ribs, and lungs.
This amazing discovery sheds light on how fish gradually adapted to life on land through intermediate forms like Tiktaalik. Referencing the Tiktaalik website, this fossil was found on Ellesmere Island in Nunavut, Canada by a team from the University of Chicago and the Academy of Natural Sciences.
Tiktaalik gives us a snapshot into the evolution of limbs from fins and the eventual movement of vertebrates onto land.
Panderichthys
Panderichthys is an extinct fish species that lived around 385 million years ago in the Late Devonian period. It had many tetrapod-like features and is considered a transitional fossil between fish and early tetrapods.
Some important features of Panderichthys include its flattened skull, flexible neck, and relatively large forefins. According to the New Yorker, these forefins were equivalent to the front legs of a tetrapod and likely had large bony elements inside them rather than fish-like rays.
Panderichthys also had nostrils and eyes positioned more dorsally on its head compared to fish. These adaptations likely helped it make brief forays onto land. While not a direct ancestor, Panderichthys helps demonstrate the sequence of evolutionary changes as vertebrates transitioned from water to land.
Acanthostega
Acanthostega is an early tetrapod that lived about 360 million years ago during the Late Devonian period. This creature is one of the first vertebrates to possess recognizable limbs with digits rather than fish fins.
According to research published in Nature, Acanthostega had eight fingers and eight toes connected by webbing. This mixture of fish and tetrapod characteristics allowed it to prop itself up and drag itself through shallow water or muddy flats.
Acanthostega had gills, so it was still reliant on water for respiration. However, it also had sturdy limb bones, wrists, and ankle joints. So while not fully terrestrial, Acanthostega represents an important transitional form displaying the gradual evolution of fish to tetrapods with full weight-bearing limbs.
Development of Limbs and Lungs
From fins to feet
The evolution from fish to tetrapods (four-limbed vertebrates) required adaptations for living on land, including the development of limbs and lungs. According to the fossil record, the first tetrapods evolved from lobe-finned fish around 385 million years ago during the Devonian period.
Fish fins are supported by long, thin bones while tetrapod limbs have much more robust and shortened limb bones able to support the body weight against gravity. The transition from fins to feet involved incremental changes over millions of years as some lobe-finned fish spent more time swimming in shallow water and even venturing onto land.
In particular, the fleshy fins of Eusthenopteron, an extinct lobe-finned fish from the Late Devonian, show evidence of these adaptations. The fins contain basic precursors of arm and leg bones such as the humerus, radius, ulna and even primitive wrist bones.
Researchers have described it as a “fishapod” representing an intermediate between fish and tetrapods.
Adapting the respiratory system
As early tetrapods spent more time out of the water, major adaptations were needed to breathe air instead of obtaining oxygen from water via gills. Gills do not function well in air.
Many Devonian lobe-finned fish species already had both gills and a primitive air-breathing organ known as a swim bladder. Over time, the swim bladder grew more vascularized and developed into lungs. By the Carboniferous period, tetrapods has fully developed lungs and the ability to breathe air with oxygen going directly to the bloodstream.
Interestingly, the lungs of modern amphibians still retain some of the ancestral characteristics of swim bladders. For example, the smooth muscle used to inflate and deflate the bladder evolved into muscles that enable today’s amphibians to force air in and out of their lungs.
Spread of Tetrapods and Evolution of Amniotes
Radiation of early tetrapods
The ancestors of tetrapods began adapting to life on land during the Late Devonian period about 375 million years ago. The first tetrapods evolved from lobe-finned fishes over an extended period as adaptations to a terrestrial life took hold.
Early tetrapods were amphibians, still requiring water for reproduction. However, they possessed more robust limb structures and lungs which would eventually enable movement around on land.
There was an explosion of tetrapod diversity between the Late Devonian and Carboniferous periods as these pioneers radiated into various ecological niches. Some had giant sizes up to 6 meters long to be apex predators (thoughtco.com).
The ability of early tetrapods to extract oxygen from the air led to success and dominance well beyond their ancestral aquatic environments. New selection pressures saw a trend towards stronger limbs, digits, and more compact skulls suited to catching land-based prey.
Emergence of amniotic egg
A key evolutionary development was the emergence of the watertight, terrestrially-adapted amniotic egg. This vital adaptation occurred around 340 million years ago and allowed amniotes to complete their entire life cycle away from water.
The egg contained its own supply of nutrients and fluids to support embryo development. It featured extraembryonic membranes which facilitated respiration and waste filtration.
The first fully terrestrial amniotes that hatched from these durable eggs began thriving on land over 300 million years ago. They diversified rapidly into numerous lines including synapsids, sauropsids, anapsids and eureptiles.
Amniotes became the ancestors to later mammal species, as well as modern reptiles, dinosaurs and birds. In fact, over 90% of all living tetrapod vertebrates are amniotes, highlighting the significance of this key evolutionary milestone (
The Path to Mammals and Humans
Synapsids lead to mammals
Around 300 million years ago, a group of reptiles called synapsids first appeared during the Carboniferous period. Synapsids were distinguished by having a single opening behind each eye socket, allowing for stronger jaw muscles.
Over millions of years, synapsids evolved many mammalian features, such as fur and warm-bloodedness. Around 200 million years ago, the first proto-mammals emerged from advanced synapsids called therapsids.
These mammal-like reptiles eventually gave rise to true mammals about 165 million years ago during the Jurassic period.
Early mammals were small, nocturnal creatures living in the shadow of dinosaurs. But when the dinosaurs went extinct 65 million years ago, mammals underwent an explosive diversification and grew to fill many ecological niches.
Over time, mammals evolved many complex features like placental development, opposable thumbs, and large brains.
Rise of primates and hominins
About 60 million years ago, a group of tree-dwelling mammals called primates emerged, characterized by forward-facing eyes, dexterous hands, and larger brains. Primates likely evolved these traits to better navigate tree canopies when searching for fruit and insects.
About 25 million years ago, a subgroup of primates called apes appeared and eventually diverged into the ancestors of modern great apes and humans.
The first primitive hominins, which includes modern humans, appeared between 5 and 8 million years ago in Africa. Early hominins developed bipedal walking and made simple stone tools. Brain size increased rapidly, and by 200,000 years ago, modern Homo sapiens emerged.
Homo sapiens’ large, complex brains facilitated specialized expertise, sophisticated communication, and social cooperation, which enabled our species to thrive around the world.
The evolution from our fish ancestors to Homo sapiens was a long process spanning hundreds of millions of years. Many remarkable adaptations arose along the way, ultimately leading to the appearance of modern humans with large, highly advanced brains.
Our fishy evolutionary origin reminds us of our deep connection with all living creatures on Earth.
Conclusion
By tracing the evidence left by ancient fish, amphibians, reptiles and primitive mammals, we can chart an incredible evolutionary journey from water to land. While many details remain shrouded in prehistory, it’s clear that humans owe a great debt to the fish we once were.
Our anatomy still carries clues of this heritage, from the similar pattern of muscles in our limbs to the gill-like structures of our embryos. Each discovery brings us one fin stroke closer to comprehending the deep roots of our family tree.
