Final answer:
The estimated cranial capacity of WT 15000, the Nariokotome Boy, was 900 cc, which would have remained the same in adulthood.
Step-by-step explanation:
The cranial capacity of WT 15000, also known as the Nariokotome Boy, is estimated at 900 cc (cubic centimeters) and would have been 900 cc as an adult. The Nariokotome Boy is an important specimen of Homo ergaster and is renowned for his remarkably modern appearance despite being 1.6 million years old. As is described in the text, his adult cranial capacity is the same as his estimated juvenile cranial capacity due to being almost fully grown at the time of his death.
Cranial capacity provides an approximate size of the brain, and it's relevant to note that the average cranial capacity of H. ergaster is 1,100 cc, while modern humans average 1,400 cc. Cranial capacity can offer insights into the cognitive abilities of various hominin species and their developmental stages.