For example, the chirality of basic biological molecules all seem the same across all life forms. Mathematically and chemically speaking, a different chirality would be exactly as plausible for life.
The most likely explanation is that it does come from a single origin. Either it only started once, or the particular lineage that led to all the life we see now outcompeted other, less efficient forms of primitive life.
The second explanation is not plausible because at least we would have found some evidence, i.e., samples of dead and outcompeted life forms.
Of those that formed billions of years ago, no trace would remain, or, if it did, it would be indistinguishable to us from any non-living molecule that once swam in the primordial soup.
Does organic material that can exhibit chirality last very long after the organism dies? I thought it pretty much all breaks down to simple molecules and elements within 10 million years or so.