Simplify the following text for a Grade 8 native English-speaking person.
Original: Originally proposed in 1899 by German physicist Max Planck , these units are also known as natural units because the origin of their definition comes only from properties of nature and not from any human construct .
Simplified: Planck proposed these units in 1899 . They are also known as natural units because the origin of their definition comes only from properties of nature and not from any human construct .