The deep history of the number words.

We have previously shown that the 'low limit' number words (from one to five) have exceptionally slow rates of lexical replacement when measured across the Indo-European (IE) languages. Here, we replicate this finding within the Bantu and Austronesian language families, and with new data for the IE languages. Number words can remain stable for 10 000 to over 100 000 years, or around 3.5-20 times longer than average rates of lexical replacement among the Swadesh list of 'fundamental vocabulary' items. Ordinal evidence suggests that number words also have slow rates of lexical replacement in the Pama-Nyungan language family of Australia. We offer three hypotheses to explain these slow rates of replacement: (i) that the abstract linguistic-symbolic processing of 'number' links to evolutionarily conserved brain regions associated with numerosity; (ii) that number words are unambiguous and therefore have lower 'mutation rates'; and (iii) that the number words occupy a region of the phonetic space that is relatively full and therefore resist change because alternatives are unlikely to be as 'good' as the original word.This article is part of a discussion meeting issue 'The origins of numerical abilities'.