To answer your question, we first have to understand what a Mariner DNA element is (I assume you know about DNA).

Mariner and related DNA sequences are known as transposons (see these links: Wikipedia or nature).

There are two particular types of transposons found in cells (and human cells), RNA- based and DNA-based transposons.

RNA based transposons make a copy of themselves and insert the new copy somewhere else in the genome.

DNA transposons (like Mariner elements) actually move the transposon from one place to another.

The human genome consists of about 3,000,000,000 base pairs of DNA (divided between 23 chromosomes), and most of your cells contain two copies of this genome.

Analysis of human genomes indicates that about 45% of the human genome is derived from transposons and the vast majority of these transposons have been inactivated through mutation, that is, mutations have occurred that block the ability of the transposon to jump from place to place.

In the specific case of Mariner transposons in human, sequence analysis indicates that there are approximately 14,000 copies that amount to about 2.6 million base pairs (which is about 0.1% of the total DNA in the cell).

While Mariner elements seem to have first appeared in insects (specifically fruit flies, see here), they appear to have moved into many mammalian lineages, including the evolutionary ancestors (anthropods) of human about 80 million years ago, well before anything like humans appeared.

It appears, based on sequence analysis, that mariner elements stopped moving in the primate-human lineage about 50 million years ago [see Pace II & Faschotte, 2007 to go deeper]

So since human ancestors appear to have migrated from Africa about 170,000 to 130,000 years ago, they already had those mariner elements well before that. Let me know if things are not clear here.

[Moderator's note: The original evidence for the "Out of Africa" model for the origin of modern humans was the discovery made in 1987 that the mitochondrial DNA of all modern humans is derived from a common female ancestor who lived in Africa about 200,000 year ago. -- SM]