Depends, of course, on what one means by "consciousness" but, if you
mean the high end of the spectrum (planning ahead, rational thought,
speaking a sentence you've never spoken before) and not something as
commonplace as awareness, then "when" turns out to be an interesting
question.
One of the puzzles in paleoanthropology is the relatively static
nature of the Homo eretus culture. There were two different toolkit
styles apparent by 1.5 million years ago, but there isn't much progress
thereafter. Many suspect that the transition from HE to Hs maybe
250,000 years ago, and the flowering of culture thereafter, was due to
some substantial step up in intellectual capacity -- even though HE's
brain was already pretty large, up within the modern Hss range.
The step might well be associated with an emerging capacity for
syntactic language at the HE-Hs transition. While contemplative
consciousness probably exists even in the apes, judging from the
deception literature, adding structured language would have allowed much
more sophisticated thought -- including rational trains of inference.
There's a bit of this in the last chapter of THE CEREBRAL CODE (page
along towards the end), which you can now find at:
http://weber.u.washington.edu/~wcalvin/bk9ch11.html
That book doesn't have much paleoanthro discussion in it, however. My
serious treatment of consciousness is in HOW BRAINS THINK, chapter 3:
http://weber.u.washington.edu/~wcalvin/bk8ch3.html
and the evolution of intelligence is in the following chapter.
--
William H. Calvin WCalvin at U.Washington.eduhttp://weber.u.washington.edu/~wcalvin/