If you really want to stick with
tkz-base then the only way [that I can find] to do this is to reset your axes to be positive, and then put the labels in by hand.
\tkzInit[xmin=0, xmax=150000, xstep=10000, ymin=4, ymax=18, ystep=2]
Of course, if you do this you would need to adjust your data points so that the years are positive too. I guess you can think of them as "years since 150,000 years ago". But as noted in the comments, you might find it easier to use
tkz-euclideis more designated to geometric drawings. And your plot seems to be quite easy with PGFplots...
tkzcollection also has
tkz-fctfor plotting stuff. The
\tkzAxestuff is defined in