Fossils of two Early Jurassic mammaliaforms—Morganucodon and Hadrocodium—show that the mammalian brain did not appear in its modern form all at once. X-ray tomography indicates two evolutionary pulses in which relative brain size expanded toward mammalian levels, alongside changes in smell, touch and movement control.
That evidence helps explain the deep evolutionary background of the human brain, but it does not show that either animal was a direct human ancestor, or that one species single-handedly caused the brain humans have today. The available study also does not establish that these animals were predators, so “mammaliaforms” is the more accurate term.
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What the Jurassic fossils reveal
The key evidence comes from a 2011 study, “Fossil evidence on origin of the mammalian brain,” which examined fossil skulls of Morganucodon and Hadrocodium with X-ray tomography. Rather than preserving brain tissue, the skulls preserve the shape of the space where the brain and sensory structures sat. Those internal spaces, known as endocasts, let researchers estimate how major brain regions changed through time.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteThe authors report that relative brain size expanded to mammalian levels in two pulses. Their abstract summarizes the result this way: “We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses.” Relative size matters because it compares brain volume with body size; a larger brain relative to the body can indicate a shift in sensory processing and behavior even when absolute measurements are unavailable.
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How the two evolutionary pulses worked
First pulse: smell, touch and coordination
The initial expansion probably involved several systems at once. The study links it to improved olfaction, greater tactile sensitivity associated with body hair, and better neuromuscular coordination. These are connected demands: an animal that relies more on smell and touch must process richer sensory input, while more precise movement requires tighter communication between the brain, muscles and sensory organs.
The wording “probably” is important. The fossils support a pattern of anatomical change, but they cannot show an animal’s thoughts or behavior directly. Researchers infer likely pressures from the relationship between the endocast, sensory structures and the rest of the skeleton.
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Second pulse: a stronger mammalian sensory package
A later increase brought brain proportions closer to those seen in mammals. The enlarged regions included the olfactory bulbs, neocortex, pyriform (olfactory) cortex and cerebellum. Together, those changes point to expanded capacity for processing smell, integrating information, coordinating movement and handling more complex sensory signals.
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The study also identifies a further enhancement of olfaction at the origin of crown Mammalia. It associates that step with ossified ethmoid turbinals—delicate structures inside the nasal cavity that increase the surface area available for processing odors. This is a separate anatomical milestone from the earlier broad increase in brain size.
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What changed compared with earlier stages?
| Evolutionary stage | Evidence highlighted by the study | What it may indicate |
|---|---|---|
| Early mammaliaform condition | Smaller relative brain proportions than later mammals | Less expanded sensory and motor processing capacity, although exact behavior cannot be reconstructed from the abstract |
| First expansion pulse | Probable increases linked to olfaction, hair-associated touch and neuromuscular coordination | More demanding sensory integration and movement control |
| Second expansion pulse | Larger olfactory bulbs, neocortex, pyriform cortex and cerebellum | A brain organization closer to the mammalian pattern |
| Origin of crown Mammalia | Ossified ethmoid turbinals and further olfactory enhancement | Greater specialization for detecting and processing odors |
The surfaced study abstract does not provide a complete set of brain-volume measurements, a sample-size breakdown or a species-by-species numerical table. The comparison is therefore anatomical and relative, not a claim that one fossil had a precisely stated percentage increase over another.
Why this matters for the human brain
Humans inherited a brain built from mammalian systems that accumulated over a very long sequence of changes. The Jurassic fossils capture an early part of that sequence: sensory regions and coordination networks were becoming more prominent before anything resembling the human brain existed.
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That makes “paved the way” a useful broad metaphor, not a statement of direct ancestry. Morganucodon and Hadrocodium belong to early mammaliaform branches. The study does not demonstrate that either one sits on the unique line leading to humans. Instead, their anatomy helps identify developmental and ecological pressures that were important across mammalian evolution.
The same distinction prevents a common mistake in popular explanations. A larger relative brain is not equivalent to humanlike intelligence. Brain regions can expand for different reasons, and fossil endocasts cannot reveal language, consciousness or social behavior. They show the physical space and proportions available to neural tissues.
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How scientists can infer brains from stone
- Tomography captures the skull interior. X-ray scans reveal the three-dimensional shape of the braincase without cutting through the fossil.
- Researchers build an endocast. The reconstructed cavity records the approximate outlines of the brain, olfactory bulbs and other structures that pressed against the bone.
- Proportions are compared. Brain size is assessed relative to body size and compared across early mammaliaforms and later mammals.
- Anatomy is interpreted alongside function. Enlarged regions and nasal structures suggest sensory or motor changes, but the conclusions remain probabilistic rather than direct observations of behavior.
What the evidence does not show
- It does not prove that either fossil animal was a predator; the cited abstract does not establish diet.
- It does not identify a single species as the direct cause or exclusive ancestor of the human brain.
- It does not provide a complete numerical timeline of brain growth for every early mammal.
- It does not allow claims about humanlike intelligence, consciousness or behavior from brain proportions alone.
The strongest conclusion is narrower and more useful: mammalian brain evolution involved staged enlargement and reorganization, with early changes tied to smell, touch and coordination, followed by further sensory specialization near the origin of crown Mammalia. Those steps formed part of the deep background from which later mammalian, primate and eventually human brains evolved.
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