You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Somalis

Context:
According to mtDNA studies by Holden (2005) and Richards et al. (2006), a significant proportion of the maternal lineages of Somalis consists of the M1 haplogroup. This mitochondrial clade is common among Ethiopians and North Africans, particularly Egyptians and Algerians. M1 is believed to have originated in Asia, where its parent M clade represents the majority of mtDNA lineages. This haplogroup is also thought to possibly correlate with the Afro-Asiatic language family:

Question:
When did Richards publish his mtDNA research?

2006