Question: I want to post my manuscript to bioRxiv before submitting onto formal journals, but I still have several concerns:

1. Can I claim I am the first for the discovery by posting my findings on bioRxiv ?
2. Similarly, is it a feasible way to prevent others from publishing ealier than me on the same topic ?
3. Are there any risks of having my research scooped after posting my manuscript on bioRxiv ?

Thanks and Best regards,

Michael

Answer: 
(1) I discussed this question recently with few colleagues from biomedical field, and, as far as I can see, major part of the field do not accept a non-reviewed experimental findings as a proof of discovery. This is mostly for two reasons: first, there is no single preprint server in biology. There are BioRxiv, F1000, PeerJ, ASAPbio, Arxiv q-bio, and probably more. And all these papers are not indexed in the main paper bases, like PubMed or WoS. So to keep up with the "pre-publications" one will need to check every single pre-print server. And this is too much. Second problem is the fear that if the field will start to acknowledge the priority based on the claim, without checking the rigor of experimental design, it may promote the sloppy data handling, because researchers will rush to put their flag everywhere. And we are talking about the very populated field, where many sub-fields have a clinical relevance or otherwise related to highly sensitive problems of human/animal health and well-being.

(2) Probably no, but this is the risk that one takes in research when he/she is going to conference, submitting grant application, and etc. The question is whether this risk provides some benefits in return.

(3) I still think that biomedical field, in general, would benefit from fast communications of the research finding behind the closed doors (i.e. without making scientific statements to the public). Even from the personal perspective, it is a way to faster position yourself as a researcher among other colleagues.
Putting immature article before peer review is not proof of discovery.
If yes, we can easily scoop other's findings by posting quick, short and sloppy article in Preprint server (after knowing their data for example in some research conference) .

Also, the benefit of Preprint server is that researcher can publish there data quickly before long peer review and publication process.
However, in reality, most of the author post their articles in preprint sever when they noticed that some other group were also working on the same topics and wanted to insist their priority. This is quite ugly and selfish motivation.

All paper needs to go through peer review when the paper needs to be polished. 
Even though that is time-consuming, this process is equally required for all academic paper. 
Therefore, posting article in preprint server after knowing some competition is dirty scooping.