Final answer:
Hominids, the first organisms to resemble humans, appeared around 2.5 million years ago based on fossil evidence. In contrast, trilobites, the earliest arthropods, emerged over 500 million years ago and became extinct around 250 million years ago.
Step-by-step explanation:
According to fossil evidence, the organism that first appeared around 2.5 million years ago is the hominid. The first hominid fossils appear in the late Miocene, between 10 to 5 million years ago (MYA). Roughly between 7 MYA and 4 MYA, hominids began to transition from arboreal (tree-dwelling) to terrestrial life.
Contrastingly, trilobites, the well-known earliest arthropods, appeared significantly earlier in the fossil record. Trilobites first emerged more than 500 million years ago during the early Cambrian period and were a dominant part of marine ecosystems. They became extinct approximately 250 million years ago, at the end of the Permian period.