Brownian motion tutorial script

[Manel Aitelhadj]

Hi, 

I am currently doing the BM tutorial on RevBayes (https://revbayes.github.io/tutorials/cont_traits/simple_bm.html) and I am having trouble understanding one particular command. 

T <- readTrees("data/primates_tree.nex")[1]

What does the [1] mean here? knowing that if I don't put it the dnPhyloBrownianREML won't work. it doesn't seem to be any explanation on that.

Thank you in advance, 

Manel AIT EL HADJ,
Ph.D candidate in bioinformatics and computational biology
Carolin Kosiol’s lab
Sir Harold Mitchell building 
University of St Andrews 
St Andrews
Fife
KY16 9TH
UK.