Homo sapiens, our species, is known for having progressively increased our brain volume over time. Furthermore, this is a trend generally observed in our group within the primates, the genus "Homo" (where we belong).
For example, Homo erectus, who first evolved in Africa about 2 million years ago (according to the Smithsonian Museum of Natural History), had an average endocranial volume capacity of 950 cm³ [1], while early Homo sapiens, who first evolved only about 300.000 years ago (Smithsonian Museum of National History), had almost 1.500 cm3 [2].
Modern humans, like you who are reading this, have an average brain volume of about 1,300 cm³. In other words, our modern brains are about 13% smaller (see news in BBC). What happened to us?
There are several hypotheses to explain this, but it seems that none of them can be fully proven yet. Let's analyze two of them:
Invention of symbolic language: About 100,000 years ago, our ancestors started using a more sophisticated symbolic language. This potentially optimized our hardware, turning our brains smaller but better organized and more efficient, like a modern computer [2]. Other authors criticize this argument and point toward a shrinkage not necessarily correlated with language invention [3].
Climate change: According to another study, it was climate change, and not language, that drove our brains to become smaller [4]. According to this hypothesis, the shrinkage started later, about 17,000 years ago, since the end of the last ice age. This was because there is a correlation between warmer climates and decreasing brain size, probably for metabolic reasons to optimize heat loss.
A question that you may be asking is: does a smaller brain make us less smart? Well, not necessarily. Similar to comparing an old bulky computer with a modern stylized one, it is the wiring that matters, not its overall dimensions (more about this on BBC).
In conclusion, this is a fascinating topic, but quite complex. Our fossil record and lineage are very patchy and maze-like, with many fossils but not clear and detailed relationships between one another in many cases.
Don't forget that our evolution, rather than being a simple linear transition from an ape-like being that became a person, is a very bushy tree with several unconnected pieces. This diagram shows only the last 3 million years or so, and you can already see several different species coexisting:
There are still many things to uncover. What do you think our brains will be like in the future?
References
[1] G. P. Rightmire, “Homo erectus and Middle Pleistocene hominins: Brain size, skull form, and species recognition,” J. Hum. Evol., vol. 65, no. 3, pp. 223–252, 2013, doi: https://doi.org/10.1016/j.jhevol.2013.04.008.
[2] I. Tattersall, “Endocranial volumes and human evolution,” F1000Research, vol. 12, pp. 1–18, 2023, doi: 10.12688/f1000research.131636.1.
[3] J. DeSilva et al., “Human brains have shrunk: the questions are when and why,” Front. Ecol. Evol., vol. 11, no. June, pp. 1–7, 2023, doi: 10.3389/fevo.2023.1191274.
[4] J. M. Stibel, “Climate Change Influences Brain Size in Humans,” Brain. Behav. Evol., vol. 98, no. 2, pp. 93–106, 2023, doi: 10.1159/000528710.
Cover image source: https://www.luxuriousmagazine.com/human-brain-sizes-are-shrinking/
