A really big problem in studying the evolution of the brain is that we have very little idea how the organ develops. So this paper by Bystron and colleagues in Nature Neuroscience is pretty interesting:
We describe a distinctive, widespread population of neurons situated beneath the pial surface of the human embryonic forebrain even before complete closure of the neural tube. These 'predecessor' cells include the first neurons seen in the primordium of the cerebral cortex, before the onset of local neurogenesis. Morphological analysis, combined with the study of centrosome location, regional transcription factors and patterns of mitosis and neurogenesis, indicates that predecessor cells invade the cortical primordium by tangential migration from the subpallium. These neurons, described here for the first time, precede all other known cell types of the developing cortex.
The question is whether these early-migrating neurons, which make it into the developing cortical regions before any local neurons originate, might be essential to laying down pathways that later develop.
There's some clever work detecting gene expression in these neurons to determine if they belong to one or another already-known neural population (they don't). And they're not like any early neurons so-far observed in any other species:
No equivalent of predecessor cells has been described in any other species. In rats there is evidence that the earliest neurons migrating tangentially to the cortex arise from the VZ of the lateral ganglionic eminence at embryonic day 12.513, when neurogenesis has already started in the dorsal telencephalon. In contrast, human predecessor cells invade the cortical primordium from the basal telencephalon at CS12, 1 week before the appearance of the lateral eminence. A re-examination of early neurogenesis in rodents and other species is urgently needed to determine whether predecessor cells are unique to the human brain.
So they represent an early step in a "developmental cascade" in the cortex, and they are possibly primate- or even human-specific.
Bystron I, Rakic P, Molnár Z, Blakemore C. 2006. The first neurons of the human cerebral cortex. Nature Neurosci 9:880-886. DOI link