From here:
It also serves to remind us of the default setting for human beings. Looking at that picture does not make me think that it's men who might mess up the oceans but that it's human beings.
If the cartoonist had depicted women, I would read it as an argument about women only. Because that's how the default is set up in our minds.
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Added later: The picture also uses a race default which got me, too. I think the default race depends on the country one lives in and its racial makeup. The default gender does not.