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For discussion of the difference between the origin of the cancer and the common ancestor of extant cancers, see Rebbeck et al (2009) Note that Murgia et al's (2006) estimate was based on divergence of tumors from each other, and so only only estimates the time of the common ancestor. Especially note that Murgia et al say:
Whether this time period represents the time the tumor first arose or whether it represents a later bottleneck in the tumor's dispersion as a parasite cannot be resolved.