Human beings are surrounded by diatomic oxygen in the atmosphere. It enables them to breathe, ha 丷, and is necessary to sustain terrestrial life. A diatomic molecule of oxygen O2 is comprised of two atoms of the chemical element oxygen O which has the atomic number 8, bā 八.
A good example of the use of the ha 丷 symbol can be seen on the Lascaux safety lamp.
It seems to me that whoever devised the symbol ha 丷 understood the molecular structure of diatomic oxygen.
Ha 丷 does not appear in the Shuowen Jiezi. However, the character bā 八 does. Yellowbridge gives ha 丷 as a variant of bā 八¹.
The area where the Lascaux safety lamp was found has been dated to 18,735 years, indicating that the character is at least that old2.
1. Radical number 12: Yellowbridge: List of bushou from the Kangxi Zidian: http://www.yellowbridge.com/chinese/radicals.php
2. Chantal Jègues-Wolkiewiez: Datation du Puits de Lascaux, September 2006: http://www.archeociel.com/datation_du_puits_de_lascaux.htm