"Wild" watermelons (the original species and also those that are closely
related) are full of cucurbitacins and are bitter and toxic, to varying
degrees. See:

"Wild watermelons, called citron in Africa, have a spherical, striped fruit,
and white, slightly bitter or bland flesh. The pale flesh tastes like the
rind of a typical watermelon."
From: http://www.nmsu.edu/~molbio/plant/watermelon.html

"The tetracyclic triterpenoid cucurbitacins, bitter substances of the
Cucurbitaceae, are highly toxic to mammals with intraperitoneal median
lethal dose values for pure Cucs in the mouse of 1.2 mg Cuc A/kg; 1.0 mg Cuc
B/kg; 6.8 mg Cuc C/kg and in the rat 2.0 mg Cuc A/kg (1)."
From: http://www.umresearch.umd.edu/CGC/cgc6/art/CGC6-37.html

Many plant species have been around, mostly unchanged, for millions of
years. That does not mean that we or our ancestors ever took them as a
significant portion of the diet, that they are/were healthful, or that they
were even eaten at all. Take cycads
(http://plantnet.rbgsyd.gov.au/PlantNet/cycad/toxic.html) and ferns
(thiaminase), for example.
There is also no reason to assume that just because a species exists now
(even solely) in Africa, that it did not "initially" evolve on any other
continent, or that genetic drift did/does not occur within that family or
genus across continents.
Many plant genera cover the globe and more than a few *specific* species
exist within a latitude range all the way 'round (panarctic, pantropical,
etc.). Certainly, locality (and thus local evolution) exists, but it is not,
in general, nearly as limiting as many on this list seem to assume.

Ellie

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