If you needed proof that Earth already hosts something like alien life, look no further than the octopus. Beneath its soft, boneless body lies a circulatory system so strange it seems borrowed from science fiction: three hearts and blood tinted blue.
Why Three Hearts?
Two of an octopus’s hearts are dedicated entirely to pumping blood through its gills, pushing it past thin membranes to pick up oxygen from the water. The third, larger heart then takes that oxygenated blood and sends it circulating to the rest of the body — muscles, organs, and that famously flexible mantle.
There’s a catch, though. The main heart actually stops beating when the octopus swims. Jetting through the water is so physically demanding that it overrides normal circulation, which is part of why octopuses prefer crawling along the seafloor to swimming. Speed comes at a real physiological cost.
Blue Blood, By Design
Human blood is red because it relies on haemoglobin, an iron-based molecule, to ferry oxygen. Octopus blood uses a copper-based molecule instead, called hemocyanin. When bound to oxygen, hemocyanin turns a pale blue, giving octopus blood its distinctive color. Copper-based blood is far less efficient at transporting oxygen than iron-based blood, especially in warm conditions. But it works remarkably well in cold, low-oxygen water — exactly the environment many octopus species evolved to survive in, including the icy depths of the ocean floor.
Nine Brains, Not One
The hearts and blood are just the start of the octopus’s strangeness — it doesn’t have a single brain either. It has nine. A central brain sits between its eyes, but each of its eight arms contains its own cluster of neurons, or ganglion, effectively acting as a mini-brain.
These arm-brains aren’t just passing along orders from headquarters. Each one can process sensory information and trigger movement on its own, meaning an arm can react to a touch or grab an object even when it’s cut off from the central brain. In experiments, severed octopus arms have been shown to continue reaching and recoiling from stimuli entirely on their own.
This decentralized setup means the central brain doesn’t micromanage every motion. Instead, it can hand off tasks — like exploring a crevice for food — to an arm and let that arm figure out the details itself. It’s less like one mind controlling a body and more like a small committee of semi-independent minds working in loose coordination.
A Different Kind of Intelligence
The octopus’s strange plumbing is just one part of a much bigger story: this is an animal that evolved intelligence on a completely separate branch of the tree of life from vertebrates. Our last common ancestor with octopuses lived roughly 500 million years ago, long before brains as we’d recognize them existed.
And their intelligence is distributed almost as strangely as their circulation. About two-thirds of an octopus’s neurons live not in its brain but in its arms, allowing each arm to taste, touch, and even make certain decisions semi-independently of the central brain. It’s a body plan that lets scientists ask a genuinely alien question: what does it feel like to think with eight partly autonomous limbs?
That combination — copper blood, triple hearts, and nine brains spread throughout the body — makes the octopus one of the closest things we have to studying extra-terrestrial intelligence, without ever leaving our own oceans.
