For a brief period in late 1968, early 1969, Lowell George was a member of The Mothers. He can be heard on the album 'Weasels Ripped My Flesh' and the first disc of 'You Can't Do That on Stage Anymore, Vol. 5', which included a version of "Here Lies Love" with Lowell as lead vocalist.
There isn't much photographic evidence of Lowell with Frank, but this photo has just been released by a member of Lowell's family.
And here's another that's recently surfaced, from early 1969. From left to right: Jimmy Carl Black, Bunk Gardner, FZ and Lowell
See also: http://bangnzdrum.blogspot.co.nz/2012/06/when-lowell-george-was-future-of-rock-n.html