Insects are intentionally excluded. If needed, the last "life-form" would develop ways to sustain itself, including eating each other. If the options are confusing, go with your gut. Examples: horse is a land animal; penguin is a bird; apple tree is a plant.
ETA: Sustaining itself includes its respiratory ability - it would evolve or adapt as needed, too. I got an error message when posting this and didn't realize this entry went through.
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