Figure 8: Evolutionary trees show relationships between phyla, but
not when and how the body plans diverged. This is because at the moment
where lineages split and begin to change independently--represented by the
branch points on an evolutionary tree--each branch had precisely the same
body plan, and it may have been many millions of years before independent
body plans arose. Consider this tree, showing a swordfish, a fly and
Marilyn Monroe. Marilyn is more closely related to the swordfish than to
the fly, but when the two lineages diverged--over 400 million years
ago--they shared almost all of their morphological characteristics and
differed in very few. This branching pattern cannot in itself tell us when
the characteristics unique to the fly, fish or human lineage actually
arose, and for this scientists turn to the fossil record. (Illustration by
Linda Huff.)
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