Linguistically, I don't think d and p are close at all. But a sibling comment also links to the Wikipedia entry for this common fallacy.

what? linguists would universally consider d and p extremely similar, they're near identical plosives

Plosives are a big category. "k" is also a plosive, but does not sound anything like a <d> or <p>. d and p are plosives but are made using completely different parts of the mouth (dental vs labial). Perhaps some linguist will consider them "close" but "near identical" is almost certainly wrong, but I am happy to be proven wrong with a citation.

Quickly glossing over index diachronica, there's no common sound change of p<->d, but p<-b->m with the medial b has instances. (sorry for the syntax, no idea how to express this)

Alveolar for /d/, at least the way I say it.

both you and the comment I'm replying to are hilariously hackernews pedantic

consider all the sounds humans can produce

now ask yourself

1) amongst them, if p and d are close (there is no world in which the answer to that is "no")

2) if there are real life examples of languages shifting between them or using them interchangeably (there are, papa and dada being a prime example)

Of course, I did qualify my initial comment with "linguistically", the intention was to be pedantic.