There are many lower birth like dog, pig, and etc.
So which is the lowest birth possible. And what kind of a crime do I have to commit to attain such birth.
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- Now after having suffered the torments inflicted in the hells, the evil-doers pass into animal bodies. 2. Criminals in the highest degree enter the bodies of all plants successively. 3. Mortal sinners enter the bodies of worms or insects. 4. Minor offenders enter the bodies of birds. 5. Criminals in the fourth degree enter the bodies of aquatic animals. 6. Those who have committed a crime effecting loss of caste, enter the bodies of amphibious animals. 7. Those who have committed a crime degrading to a mixed caste, enter the bodies of deer. 8. Those who have committed a crime rendering them unworthy to receive alms, enter the bodies of cattle.
(Vishnu Smriti XLIV)
Therefore, the sinners of the highest order enter plants bodies. So, plants lie at the lowest stage in the hierarchy.
We also find the phrase "from Brahma to the blade of grass" to describe entire creation in one go in various Hindu scriptures.
For example, from Devi Bhagavatam:
All the things from Brahmâ down to a blade of grass, the Vaisnavas regard as very trifling; even if they get Sâlokya, Sârsti, Sâyujya and Sâmîpya of Hari, they do not care a straw for that. Without serving Hari, they do not accept those things, even if those are thrust on them
Also mentioned in this commentary (on Bhagavad Gita) by Swami Shivananda:
Another unmanifested Eternal refers to Para Brahman, which is distinct from the unmanifested (primordial Nature), and which is of quite a different nature. It is superior to Hiranyagarbha (the creative Intelligence) and the unmanifested Nature because It is their cause. It is not destroyed when all beings from Brahma down to a blade of grass are destroyed
So, if Brahma is the highest life form, blade of grass or generally plants are the lowest possible life forms.