To unlock Marcus Garvey, you must survive 6 expeditions, place no worse than 3rd, and have at least 2 expedition members fully leveled.

Alexandra David-Neel
I first successfully did it with Alexandra David-Neel. The reason I used her is because the thing that can make getting Marcus Garvey tricky is the propensity for your expedition crew to gow crazy and run amok. I've found that this is much less likely to occur with animal companions, they seem much more loyal, don't get alcoholism (as far as I can tell), and the careerist perk is uneccessary.

The thing to be careful about with Alexandra is with the first couple expeditions. She can be a little squishy until you upgrade those animals so it's important to explore as much as possible; remember, you'll need to explore at least 12 regions to get Marcus anyway! It is VERY important to be mounted on that ox as much as possible for that extra die.

Once you have experienced animals, getting into fights with wild animals is a good idea so you can get teeth to trade to beastmasters in town and swell your ranks. Keep your party full with people until you can replace them with superior animals. It's a good idea to get people with red dice as those work very well with Alexandra's natural green dice. Blue dice are practically worthless here. The bullet hoarder perk is extremely useful for pairing with shields and will let you not miss those red dice members.

The main downside to Alexandra is having low carrying capacty (non-pack animals carry nothing). So it's particularly important to upgrade your pack animal between treks as well as have those humans around to carry things. It's even worth getting the strength perk for the extra two slots. Because you need to loot shrines as much as safely possible to make sure you place at least third, the natives with help you so I didn't bother with any perks dealing with natives.

Marcus Garvey starts with:

• Authoritative Leader - can have one additional trek member, but has lower max sanity.
  
• Persian Translator
  
• Native Scout
  
• Donkey
  
• 2 whisky
  
• 3 rope
  
• 5 machete
  
• 5 extra bullets
  
• 5 shovel