A remarkable property of the contralateral organization is that it is present in every vertebrate. Even the most distant clades agnathans possess an optic chiasm, and even the skull impressions of early vertebrates from the Ordovician show the presence of an optic chiasm: this idea was worked out by Kinsbourne. There is molecular evidence for the inversion hypothesis in almost all groups of deuterostomes. It is not known, however, what exactly was the selective pressure that caused the inversion. Twisting and asymmetric development are well known from other deuterostomes  such as Hemichordata, Echinodermata, Cephalochordata and Tunicata. Turning in the side or upside-down also occurs frequently in these clades.
A remarkable property of the contralateral organization is that it is present in every vertebrate. Even the most distant clades — agnathans — possess an optic chiasm, and even the skull impressions of early vertebrates from the Ordovician show the presence of an optic chiasm: this idea was worked out by Kinsbourne. There is molecular evidence for the inversion hypothesis in almost all groups of deuterostomes. It is not known, however, what exactly was the selective pressure that caused the inversion. Twisting and asymmetric development are well known from other deuterostomes — such as Hemichordata, Echinodermata, Cephalochordata and Tunicata. Turning toward the side or upside-down also occurs frequently in these clades.
R
agnathans
— agnathans —
A
—
R
in
toward