Add abstract part one

section-ancient-metagenomics.qmd

@@ -20,10 +20,15 @@ In this chapter, we will show you how to prepare your sequencing data and subseq
 
 ## [Authentication](authentication.qmd)
 
+When exploring composition of metagenomic samples, validation of the detected organisms represents a particular challenge, because it may happen that the detected organism may have have the following characteristics
 
-<!-- TODO -->
+1. Was mis-identified (the DNA belongs to another organism than initially thought)
+2. Has a modern origin (for example, lab or sequencing contaminant)
+3. Is of exogenous origin (for example, an ancient microbe that entered the host post-mortem). 
 
-_TBC_
+In this case, authentication analysis needs to be performed to demonstrate that the detected organism is truly present in the metagenomic sample and is of endogenous and ancient origin.
+
+In this chapter, we explain how to recognise that a detected organism was mis-identified based on breadth / evenness of coverage, how to validate findings using alignments and assess multiple mapping quality metrics, how to detect modern contaminants via deamination profile, DNA fragmentation and post-mortem damage scores.
 
 ## [Contamination](contamination.qmd)