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> A study of tadpoles suggests the hypothalamus started its evolutionary journey with them:
> 
> https://advances.sciencemag.org/content/7/18/eabf7452?utm_campaign=toc_advances_2021-04-30&et_rid=17445044&et_cid=3757524
> 
> "Abstract
> The hypothalamus coordinates neuroendocrine functions in vertebrates. To explore its evolutionary origin, we describe integrated transcriptome/connectome brain maps for swimming tadpoles of Ciona, which serves as an approximation of the ancestral proto-vertebrate. This map features several cell types related to different regions of the vertebrate hypothalamus, including the mammillary nucleus, the arcuate nucleus, and magnocellular neurons. Coronet cells express melanopsin and share additional properties with the saccus vasculosus, a specialized region of the hypothalamus that mediates photoperiodism in nontropical fishes. Comparative transcriptome analyses identified orthologous cell types for mechanosensory switch neurons, and VP+ and VPR+ relay neurons in different regions of the mouse hypothalamus. These observations provide evidence that the hypothalamus predates the evolution of the vertebrate brain. We discuss the possibility that switch neurons, coronet cells, and FoxP+/VPR+ relay neurons comprise a behavioral circuit that helps trigger metamorphosis of Ciona larvae in response to twilight.
> 
> ***
> 
> "The hypothalamus has long been considered to be an “ancient” region of the vertebrate brain. It is found in all vertebrates, from jawless fishes to humans (9–11). A homologous area is also thought to occur in invertebrate chordates such as cephalochordates. The hypothalamus controls homeostasis, metabolism, and reproductive functions through a variety of intricate interconnecting neural circuits. Previous studies suggested that coronet cells in the Ciona sensory vesicle correspond to a “proto-hypothalamus” and are homologous to dopaminergic neurons in the vertebrate hypothalamus. More recent studies show that coronet cells also have nondopaminergic neurosecretory activities, raising the possibility that cellular subfunctionalization produced multiple specialized cell types in the vertebrate hypothalamus.
> 
> "Here, we compare the expression of key marker genes and single-cell whole-transcriptome profiles in the Ciona sensory vesicle and mouse hypothalamus. These studies suggest that coronet cells are not the only rudiment of the vertebrate hypothalamus. We present evidence for additional similarities, including switch neurons (mammillary nucleus), FoxP+ relay neurons (RNs) (magnocellular neurons), and VP+ and VPR+ RNs (arcuate nucleus). These studies suggest that proto-vertebrates had a sophisticated hypothalamus with multiple cell types. We propose that a major function of the Ciona proto-hypothalamus is to trigger the onset of metamorphosis in response to twilight, similar to the regulation of photoperiodism by the saccus vasculosus of nontropical fishes.
> 
> ***
> 
> "The similarities of coronet cells and associated neurons in Ciona with different cell types in the vertebrate hypothalamus suggest that the simple brain of Ciona contains a complex proto-hypothalamus. Previous studies identified coronet cells as a putative rudiment of the vertebrate hypothalamus. Here, we provided evidence for additional homologies, including switch neurons and three different RN lineages, FoxP+, and the sister lineages VP+ and VPR+.
> 
> ***
> 
> "Regardless of function, our evidence for multiple hypothalamic cell types in Ciona suggests that the apparently simple sensory vesicle has an unexpectedly sophisticated blueprint for the evolution of the complex vertebrate brain. Future studies will leverage the vast explosion of single-cell datasets to explore the origins of the neuronal cell types of different parts of the brain."
> 
> Comment: Since present stages of evolution are all based on past designs, this study of the earliest forms leading to what is contained in our present brains is not surprising. God pre-plans His stages