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Neurulation

Neurulation

Neurulation is the embryological process which forms the precursors of the central nervous system and occurs after gastrulation has established the three primary cell layers of the embryo: ectoderm, mesoderm, and endoderm. In humans, the majority of this system is formed via primary neurulation, in which the central portion of the ectoderm—originally appearing as a flat sheet of cells—folds upwards and inwards, sealing off to form a hollow neural tube. As development proceeds, the...

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Evolution shapes the features of organisms over time, ensuring that they are suited for the environments in which they live. Sometimes, selection pressure leads to the rise of similar but unrelated adaptations in organisms with no recent common ancestors, a process known as convergent evolution.

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Scaling and Neuronal Counts Evolutionary Dynamics across Amniotes.

Gabriele Sansalone¹, Silvia Castiglione², Giorgia Girardi²

¹Department of Life Sciences, University of Modena and Reggio Emilia, Modena, Italy, gsansalo@unimore.it.

Brain, Behavior and Evolution

|March 30, 2026

Summary

Evolutionary rates of brain size and neuron numbers vary significantly across amniotes, with mammals and birds showing coordinated increases, unlike reptiles. This suggests brain-neuron scaling is specific to highly encephalized groups.

Keywords:

Brain evolution Evolutionary rates Phylogenetic regression Phylogenetic ridge regression Vertebrates

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Area of Science:

Evolutionary biology
Comparative neuroanatomy
Phylogenetics

Background:

Brain size and neuron number diversity across amniotes suggests evolutionary shifts in their scaling relationships.
Understanding these relationships is key to explaining current brain diversity.

Purpose of the Study:

To investigate evolutionary rates of brain-size vs. brain-neuron number scaling across amniotes.
To identify shifts and correlations in these scaling relationships within different clades.

Main Methods:

Analyzed brain and body size, and neuron numbers for 201 amniote species.
Applied Phylogenetic Ridge Regression (RRphylo) to assess evolutionary rates.
Utilized Bayesian and Robust phylogenetic regression for relationship analysis.

Main Results:

Identified five major shifts in neuron number evolution rates.
Mammals (Primates, Ferungulata) and birds (Galloanserae) showed positive shifts; reptiles (Testudines, Squamata) showed negative shifts.
Strong correlations between body/brain size and neuron number evolution rates found in mammals and birds, but not in Testudines (reptiles).

Conclusions:

Scaling relationships between body size, brain size, and neuron numbers differ significantly across amniotes.
Coordinated evolution of brain size and neuron numbers is characteristic of highly encephalized clades like mammals and birds.
Reptiles, particularly Testudines and Squamata, exhibit distinct and less correlated evolutionary trends.