Genes were most likely present in our last common ancestor

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A note for Lesson no. 1, "You Have One Brain (Not Three)," in Seven and a Half Lessons About the Brain by Lisa Feldman Barrett.
Some context from page 18 is:

If we find the same genes in certain human and rat neurons, for example, then similar neurons with those genes were most likely present in our last common ancestor.

The appendix adds:

...genes are not the whole story when it comes to inferring whether two animals have brain features that can be traced back to a common ancestor...

The references associated with this appendix entry are:

  • Striedter[1]
  • Striedter & Northcutt[2]

The science of comparing brains is part of homology, the study of shared evolutionary heritage. Before the era of molecular genetics, homology was defined by visual similarity or inferred similarity. This made the search for similarities and differences between animal brains like following a trail of breadcrumbs in a snowstorm. Structures that seem the same can evolve differently in different species, a process called convergent evolution. One example is the head, which may have arisen in vertebrates and insects along different evolutionary trajectories. Another example may be brain size, which seems to have grown relative to body size separately for Neanderthals and modern humans.[3]

Today, homology is more based on genetic similarities and differences (referred to as deep homology); for example, two animals can share homologous genes that produce different-looking structures.[4] Even with molecular analyses, there is a lot of guesswork involved when inferring homologies across species. From evolutionary biologist Henry Gee: “Different genes seem to be involved in the development of anatomically homologous structures, and the same genes are found, again and again, involved in the specification of entirely different structures at different times of development” (p. 49).[5]


  1. Striedter, Georg. 2005. Principles of Brain Evolution. Sunderland, Massachusetts: Sinauer Associates.
  2. Striedter, Georg F., and R. Glenn Northcutt. 2020. Brains Through Time: A Natural History of Vertebrates. New York: Oxford University Press.
  3. Bruner, Emiliano. 2017. "The Fossil Evidence of Human Brain Evolution." In Evolution of Nervous Systems, second edition, volume 1, edited by Jon H. Kaas, 63–92. New York: Elsevier.
  4. Shubin, Neil, Cliff Tabin, and Sean Carroll. 2009. “Deep Homology and the Origins of Evolutionary Novelty.” Nature 457 (7231): 818–823.
  5. Gee, Henry. 2018. Across the Bridge: Understanding the Origin of the Vertebrates. Chicago, IL: University of Chicago Press.