In the Dark Ages of Autism fifty years a prevailing theory was that the cause was post-natal emotional mis-treatment of the child by a so-called ‘refrigerator mother’. This was utter rubbish. For fans of English as a consistent language I regret to report I still put things in the ‘fridge, but that the actual Latin is refrigeratus, past participle of refrigerare “make cool or cold.” The root is frigere = cold. Not a letter ‘d’ anywhere.
A thirty year-old conjecture that had more validity was that deletions on the 15th chromosome caused Asperger’s syndrome and that deletions on the X and 16th chromosome caused autism. Things have turned out to be a whole lot more complicated:there are over 180 genes on all 23 chromosomes (and even in mitochondrial DNA) implicated in autism spectrum disabilities. One of the genes implicated in Aspergers is the GABRB3 gene which is on chromosome 15 at location q12. Mutations in GABRB3 are associated with Angelman syndrome, Prader-Willi syndrome, nonsyndromic orofacial clefts and two types of epilepsy: childhood absence 5 (ECA5) and early infantile encephalopathy 43 (EIEE43). Also implicated in Angelman syndrome are the genes UBE3A (at 15 q11.2), ATP10A (at 15q12) and OCA2 (at 15q12-q13.1). The last is currently believed to only influence skin, hair and eye color as opposed to metabolism, cognition and so on.
One very puzzling mystery was, regardless of whether it was a repeat, delete or mutation on chromosomes 15, 16, X or anywhere else, how could the genes be continuing to appear? Where was the selective pressure? To compound the enigma it was conjectured that few of the many mutations were recent. No doubt to the great relief of fellow primates everywhere and everywhen many of the troublesome genes are only found in humans. It is currently still very challenging to retrieve archaic human DNA – the limit at present seems to be 400,000 years – and that was just a small sample. M. Meyer et al., “A mitochondrial genome sequence of a hominin from Sima de los Huesos,” Nature, doi:10.1038/nature12788, 2013.
The Sima de los Huesos cave in Spain has provided parts of the skeletons of at least 28 individuals from the middle Pleistocene period. Not so long ago these would have been classified as Homo heidelbergensis, a possible ancestor to Neanderthals.
But the mitochondrial genomes were more closely related to Denisovans. This was unexpected as Denisovans are known only from a finger bone and molar found in Siberia.