Plasmodium falciparum, the causative agent of malignant malaria, is among the 
most severe human infectious diseases. The closest known relative of P. 
falciparum is a chimpanzee parasite, Plasmodium reichenowi, of which one single 
isolate was previously known. The co-speciation hypothesis suggests that both 
parasites evolved separately from a common ancestor over the last 5-7 million 
years, in parallel with the divergence of their hosts, the hominin and 
chimpanzee lineages. Genetic analysis of eight new isolates of P. reichenowi, 
from wild and wild-born captive chimpanzees in Cameroon and Côte d'Ivoire, shows 
that P. reichenowi is a geographically widespread and genetically diverse 
chimpanzee parasite. The genetic lineage comprising the totality of global P. 
falciparum is fully included within the much broader genetic diversity of P. 
reichenowi. This finding is inconsistent with the co-speciation hypothesis. 
Phylogenetic analysis indicates that all extant P. falciparum populations 
originated from P. reichenowi, likely by a single host transfer, which may have 
occurred as early as 2-3 million years ago, or as recently as 10,000 years ago. 
The evolutionary history of this relationship may be explained by two critical 
genetic mutations. First, inactivation of the CMAH gene in the human lineage 
rendered human ancestors unable to generate the sialic acid Neu5Gc from its 
precursor Neu5Ac, and likely made humans resistant to P. reichenowi. More 
recently, mutations in the dominant invasion receptor EBA 175 in the P. 
falciparum lineage provided the parasite with preference for the overabundant 
Neu5Ac precursor, accounting for its extreme human pathogenicity.